I'm nowhere near a pro contact printer, but I've done about 40 the last 2 days (small ones, 8x10 max) using a contact printer (paterson) and also a "sandwich" with 2 plain plates of "opticaly perfect" glass.
The first handful of prints I could see newton ring patterns in the glass (both mounts), at an angle.. Since I was testing exposure etc, I ignored them and I kept at it without cleaning the glass (apart from obvious marks) and after 10 prints or so the rings disappeared completely.

So maybe having a "worked out" piece of glass makes a difference ? It could amount to a residue of dust etc from the various negs that were pressed against the glass, but it made a fantastic difference...

I think that now I will "train" the glass a bit before important prints...