I don't think gma's question is relevant to ULF photography, only to enlargements from 4x5 or smaller. I say that because I have never encountered diffraction problems with contact prints in 8x10 or 11x14. In ULF, lenses are usually longer than 14 in. and exposures are often longer than 2 sec. DOF and movement are the boogie men, not grain, diffraction, or lens resolution (No offense Sally Mann!)
While in Zion NP last week I intentionally tried to bring diffraction into the equation with three identical images at f45, 64, and 90. With identical development I can't distinguish them with the 10x loupe. I gave up trying to label them by f-stop.
That said, I agree that for MF and 4x5, from film choice to enlarger f-stop there is a chain of image degrading steps that must be minimized at every step, not just one.