Same question here re anti-Newton ring glass on the bottom. That's a totally new one to me. On the other hand I've never done color enlarging.
Bob: I doubt you'll be issuing any refunds. A nice thing about the multi-resistant coated (B+W "MRC") filter glass I installed in my 35mm carrier is how easy it is to keep clean (as Drew alluded to). That coating is extremely hard too, so it reduces the chance of me buggering it up with a scratch or something. It will last extremely long. I like this stuff for 35mm because with the bigger enlargement factors every little flaw can become more of an issue. Unfortunately since I threw away the regular glass for that carrier a long time ago, I can't test to see if any of this makes any difference. On balance I'd have to agree with you it is likely just overkill in the end, particularly when it comes to enlarging sheet film.
Drew: I agree sometimes people fail to consider the entire end to end system from taking lens to print. That's why I thought this would be an interesting discussion. On the other hand this particular issue might be trivial. I don't know.
Re newton rings, my Tri-X 320 spacer works like a charm. But if I could find the right anti-newton glass I'd surely use that instead. But I only know of Focal Point. So if it is as complicated as you say (ie need different types for different formats), yikes. I wouldn't even know where to start.
Appreciate everyone's input on this so far. Very interesting.