when is this necessary and can it be checked?
what is the cost?

There is 2 processes that can be done... boring and
honing.. if you change caps or if you go to studs
you should have it done.. if you just change to studs
you may only need a honing... if going with new caps
they will need to be fitted then bored and honed.. they
can check the bores for alignment to see if you need it

Don't do it unless you need it - a bad job is worse than none.
If you do - you may need new timing setup for reduced C-C distance.

It might be easier as well to ask why you think you need it or has someone told you it needed it?

The cost is all over the place... Ive seen align bore and hone for $300 plus and although id not use just any shop I use a machine works place that does every aspect of machine work from jets to locomotives and they do great work and in Jan. with other work they charged me $180 for the align bore and hone part.

I will say that I have put in studs and went cheap and didnt
have it checked.. it did live just fine.. had a bit of wear
on 1 or 2 of the main bearings... but if you are building a
nice engine.. have it checked
EDIT
when I built my 405 W-9 I was told the mains were
bored and honed.. I couldnt even turn the crank with
the caps on.. found out later he didnt do the mains

Line boring is rare. Line honing is common. I think a couple hundred bucks Can. is about right.

The only two ways I know of checking the main bore alignment involves using a long round straight(.0000 run out!) bar that is right at .0005 to .0008 smaller than the I.D. of the main caps torque down to the proper torque specs with the bar inside the main bores, if it turn your golden, if it doesn't turn you may have a problem Verify the I.D. of each main cap to make sure there within the factory specs on I.D. and out of round The other way is to install the main bearings and crankshaft to see if it turns easily with no main seal or run out on the crankshaft Do the basics and let us know what you find The neccessity comes when you suspect a problem, don't fix what ain't broke

Correct me if I'm wrong BUT ... its rarely an alignment issue but more a "fitment" issue. Once the block is bored at the factory things should not change. However, if you swap or machine the crank and you install new bearings then you need to align hone to achieve the desired clearance. This usually involves grinding a hair off the mains then line honing to the required size.

So if you take .010 off the crank on the mains and buy an oversize bearing, that's all fine and dandy BUT those bearings may have too little or too much clearance. Line hone is how this is adjusted.

My R3 block was bored from the factory.. new block..
and the crank wouldnt turn.. had to have it line bored
and honed... same thing with my Mega block(new)

I've never had to have a stock block aligned honed to get the main bearing clearances correct when using a turned down(ground down) stock or race crankshaft, never I have had to use different main bearings brands and part numbers to get the clearances I wanted I do have all the blocks I use that have new main caps and main studs put on them aligned hone to get the main bores the size I want and straight

Well, that's "you" Cab ... because you have a rack full of bearings to chose from. But take a guy who purchases a kit or whatever and takes it to an engine builder and says "have at it". Unless they do a lot of the same brand of motor its unlikely they'll have a rack full of bearings to chose from. And rather than dick around with buying and waiting for a set to show up that fits, they just line hone the block - which potentially ends up costing the customer less.

One of the first checks you should make is during disassemble. How do the old bearings look ( wear pattern )? Next you want to check the crank for straightness ( V-blocks). All ok, Put the crank back in with the bearings you are going to use. Spin freely, you are good to go. Replacing bolts with studs usually don't warrant a line hone. Replacing caps, always. IMO, line honing or boring is often dune unnecessarily. Save yourself some $ and check it.

I forgot to mention when I have a crank ground I ask the crank grinder to cut it to the small sizes, IE factory specs on a Mopar RB crank main sizes 2.749 to 2.750 stock I ask for on a .010 under grind to have it be 2.7385 to 2.7390, not 2.7400 or larger I do stock several different brands and part numbers of both main and rod bearings so I have the choice As far as having a regular automotive machine shop align hone a block to set main bearing clearances I would rather stick with the factory align hone and use different bearings to get the clearances I want Lots of different ways to accomplish our goals

So I won't be having you build any of my motors!!

1) Stud torque is different than bolt torque
2) Bolts are installed with oil - studs with ARP lube or similar

You sure as hell DO want to line hone if switching from bolts to studs !!!

No problem there!! Check it both ways ( factory bolts and studs ) and see what the bearing clearance difference is.

Line boring is not necessary unless you have changed something.

Line honing is more often done. Usually when one or more of the bores is oversize. Cut the caps to reduce the size and hone it back to spec.

It could be checked (if you really wanted to do such a thing) by going thru the motions as if you were going to line hone it (except not cut the caps) and just stroke it a few times to make sure it's straight (and to size). Now you know it's fine. Around $150

BTW, the objective is to apply enough clamping force to keep the cap from walking in the saddle. If you apply enough clamping force to distort the cap ( change bearing clearance or bore alignment),that's too much clamping force - and that is not what studs are ment to do.

Most common issue when using studs is that studs will touch inside of cap and prevent proper setting of #3 cap for correct end play--factory bolts 'hang' in the holes and don't touch cap--so..answer is just drill holes in #3 cap out a tad --do that and usually studs will do fine but..we ALWAYS brush through when using studs and make it just right.
Boring is done when caps are changed--
Rollmaster and others offer sets that are .005 shorter when needed
I agree when you have to bore one it can be tricky and cause all kinds of trouble if not done correctly--
It is also rare when bolts will not hold what you are doing--studs make folks feel better but are a PIA back at the seal holder, with the thrust issue , and really may make you sleep better but I can't prove they are any better or that they prevent anything! Timing errors that cause detonation is what makes caps 'walk" and no stud will prevent that or make it better IMO
We do it because customers request it not because we think it is any better

Line BORING is not usually done or required, unless fitting a different set of main caps on a Block.
Line HONING is commonly performed to adjust main bearing Clrc's or straighten or bring back main Bores "round", that have subjected to different clamp loads.... ie; changing from stock Bolts to Studs.

We Line Hone everything, just a matter of procedure,because everythibg is "off" the mains and we have to square the Block Decks as well as correct the Bores perpendicular to the mains.
and NO... unless you are a dork cutting .010" off the main caps because you can't line hone properly, a shorter timing chain is NOT required.

Ya, a procedure that everyone gets to pay for because you think it is necessary every time you do block machining.