Is the plane of focus different when the grain focuser is used one-paper-thickness further away? Of course. Is it significant? It depends, but not usually.

I routinely focus with the enlarger lens "wide open" to minimize depth of focus - as much as possible - and stop down for the exposure.

If one is doing the "brick trick", tilting the easel to correct perspective, the effects of depth of focus become apparent. I have differences in paper height, from one side to the other, of as much as an inch and a half - 37 millimeters or so ... and printed "acceptably sharp" at f/16 or f/22, using the 50mm Rodenstock.

What might be interesting is the "Softeners". I purchased two 40.5mm filters from a "bargain box" - out of a curious impulse - only because they *FIT* the Rodenstock enlarging lenses. The only information I have about them is that they are marked "Softener" and they were made in Japan. *Extremely* useful in printing portraits.
Simply wonderful for removing "blemishes" - but *not good* trying to use the grain focuser with them.