Interestingly, even though it seems like Don and I are disagreeing in theory, I follow almost exactly the same practice. Since I enlarge 4x5", I try for around f:22 usually, unless I want very short DOF or high shutter speed or I'm in low light, and try not to go past f:32, though occasionally I'll go to f:45. For 8x10" and 11x14", I'll also go as far as the lens goes if I have to, which is usually f:45 or f:64, since I contact print these formats.

Michael A. Smith has written that he'll go as far as f:256 (I suspect his only lenses that go that far are longer Artars), and he contact prints exclusively.