Diffraction can be quite limiting for macro work, because it depends on the effective f-stop. A lens at f22 will have an effective f45 at 1:1, which is already limiting at 4x5 and "normal" print sizes.

Sharpness is always a compromize between DOF, diffraction and shutter speed.

I, personally, try to avoid f-stops below
f32 for 4x5
f22 for MF
f16 for 35mm
and one f-stop less for macro work.