How bad can pistons get (scratches, wear) before they can't be reused? Any pic examples?

It mostly has to do with keeping the piston rings sealed the the cylinder and piston. You need to measure the piston ring to land clearances. If the piston lands are loose, the rings won't seal.
If there is too much piston to wall clearance, if the rings do seal, they may wear out quickly.
If the only think wrong is scratches on the skirts, you may be able to clean/polish the scratches and maybe have an anti-friction coating applied to bring them back to size?

First ...and MOST important ... how is the ring groove clearance?

Well, this is a 440 and the book calls for .004" ring to land clearance. I'm getting right between .004" and .005" on all pistons. A few original rings were stuck/frozen in the grooves, and I had to use a very fine emery (400) to get the grooves clean, which apparently opened a few groove areas to .005" and is my main concern. Piston to bore clearance seems fine. There are some minor scratches (small stuff) on the skirts, and a little vertical roughness on some sides between the tops and the upper ring lands (which appear to be scratches as I see no evidence of burning). I have already smoothed a lot of this out with 600 paper, but can't afford any coating.

I Think they will be okay but wanted to check. I can post a pic if that will help.

What book says 004 ? ... my FSM says 0005 to 0015.

Wow! Maybe I have to get some better books! The HP book How To Rebuild Big-Block Mopar Engines (pg.84) and the Haynes Chrysler Engine Overhaul Manual (pg. 6-12) both say .004" for ring to land clearance.

Your measurement sounds like piston to cylinder wall clearance...no?

I stand corrected ... I read-off a spec that was high-lited in that manual.

The 440 spec for COMPRESSION RING SIDE CLEARANCE IN GROOVE is 0015 to 003.

I checked before on some NEW pistons I have ... some cast OEM-type 413 pistons and a set of forged 383 OEM-type pistons.

I could NOT put a 002 feeler gauge into either groove with a ring in place. And I also have a set of used 413 pistons .... a 002 feeler was just a tad loose.

Okay, so it sounds like my pistons are past the limit. I also noticed that my upper compression slot is slightly larger then the second one. Guess I'll start looking for some others.

Ring groove clearance is very important ...in fact critical !!

Reason is ... is if it is loose it will flutter and not be square on the bore.

you want get get crafty on the last freshen at you bore(already rehoned and file fitted the rings on same bore)for the 3rd time you can krurl the pistons skirts for 100 bucks blows them up 4 thou and takes all skirt marks out. then rehone block for proper piston to wall. it will be all fresh again and wont have to file the 5 thou over rings much

... but the rings will-still-be sloppy in the grooves

Don't forget the wrist pin bores--if they are galled you got trouble--sounds like a budget build--use plain rings--don't hone it too slick and let her go