Won't make much difference on a film that grainy anyway! I don't have a Xenar like yours, but do at times use a 105/3.5 Nikkor M (a very crisp tessar) with a 6x9 film back and high-acutance films like PanF, Efke 25, and Ektar. I don't think I'd generally want to stop down any smaller than f/22. With
75 or 105 lenses on my Pentax 6x7, even f/22 starts showing a little diffraction with such films. It
wouldn't start to show unless you're doing a 16x20 print. But ya just gotta test to see what you're
comfortable with.