... When you say "stay away from wide open", you mean shoot at like 200-225mm and not the full 250mm right? ...
No Wide open means the largest aperture.
ie f4.5 on a Nikkor ED 300mm 1:4.5 lens.
Lenses generally capture a sharper image with a smaller aperture a few stops from the widest aperture, at F8 to F16 with that lens.
F22 or what ever the smallest aperture with the smallest opening or highest F number on your lens has the widest depth of field, but again may not be the sharpest...