........where you have that done makes a difference.

If...... The performance of the motor matters to you.

Let me back up a bit here........

I had sold a roller cam to a customer a few years ago..... Car ran what it should.....several races are won...... But as things usually go....... The desire for better ET's inevitably rears its head.
The original cam was a basic bracket type cam, easy on parts, reliable.
The new cam is a big step up in ramp speed, but not much of a change to the overall theme of the cam(seat timing, lsa, etc).
It should have made a definite improvement in performance...... Not a huge difference...... But a definitely noticeable difference.
Well....... Whatever difference there might have been, it was much less than I would have expected. Only now we're wearing out springs faster than before.

I don't recall if that was run for one or two seasons, but eventually there was a major engine failure....... And now a new bottom needs to be built.
Since there's a bunch of bent valves, the heads now need to be gone through as well.

I get a call about whether it would be worth having the heads come to me of if they could just be gone through locally.
I put forth the proposition that if the local shop he plans on using is known for their high quality head porting and cylinder head prep they should be fine, but if not...... It's probably worth the trip here.

When I get them here and apart I can tell right away they don't have the valve job that was done on them when they left the original porting shop.
I make a call and learn that, no..... That isn't the valve job they came from the porting shop with. They got freshened once before...... Done by the local shop...... Just prior to the new cam going in the motor.
The light bulb came on at that moment as to why(more than likely) the new cam didn't show any gains.

If the first valve job is done so the contact area between the valve and the seat is such that the area of interface is way out on the edge of the valve....... Then the next valve job is done with a 3 angle cutter that's set up to cut to the very inside of the valve........ Then you're going to have to sink the valve until the seat angle is reestablished in its new location. This is never the hot ticket for flow.
Even if the valve seat concentricity is perfect, and the valve job is functionally 100%...... This will almost always result in less flow, and less power.

Basically........ It looks like this: