With most lenses on a D700 you will see the first evidence of diffraction around f/11 from a technical standpoint, but until you get to f/16 or so it probably makes little difference. I would consider f/16 marginal if I wanted maximum sharpness, but there are plenty of situations where DOF may be more important. For macro, I would not hesitate to use f/16 or f/22. I'd try to keep my landscape images around f/11-f/13 since the difference in DOF is relatively small.
For those with a D800, these numbers would all be reduced by about one stop.