No at F5 at 250.
I always shoot on a tripod and use a remote shutter release. (for ruler and flies)
I don't lock the mirror.
This is my light source and light box made from 2mm coroplast (for the back) and depron (foam for the front).
Mirror Lock-Up. Depending on the system/tripod/lens holding the mirror up for about three seconds then snapping the picture should help sharpness considerably if it's vibration induced softness.
Is this just a depth of field issue? Other than the legs being out of focus, the 2nd image looks fine.
Anyway, the problem may be a poorly calibrated lens. The 70-200 mm f/4 is supposed to be VERY sharp....one of Canon's sharpest zoom lenses. It should be sharper than the 70-200 mm f/2.8 model, which is already sharp. The difference between Canon and Nikon isn't sharpness. If you don't get sharp images with a Canon, you won't get them with a Nikon either.
If you shoot macros on a tripod and that studio stuff, you might find Helicon Focus useful for combining many images with a shallow DOF into one with a large DOF. That way you can get sharp images by shooting at larger apertures, no diffraction and a large DOF for your pics.
(DISCLAIMER: I haven't actually tried it, but from what I've read it's pretty good.)