Don't be afraid of using your lens away from that 'magical' two stops down optimum.

If you use a 50mm f/2.8 and usually print at f/5.6 aperture, don't be afraid of f/4, f/8, or even f/11 to get reasonable printing times. Will there be a difference? Probably. In my world, though, it's better to be at a comfortable printing time where the paper stays consistent (avoiding reciprocity failure) than having a wee bit better sharpness. Use what's practical and don't worry about it. That's my advice.

- Thomas