It has been discussed/argued to death in the past. Consensus and testing by some (but not by me) shows it makes NO difference. I use under-the-filter kit from Ilford with my D2. It's kind of necessity because otherwise, burning with different grade becomes a risky proposition.

I actually bought a large filter kit for above the condenser use but never bothered to use it in the last two years. I keep mine reasonably clean but it's far from spotless. Yet, my 16x20 print comes out pretty good. Obviously, this is not a scientific statement....

One of these days, I'll experiment by scratching the heck out of mine and do a comparison.