I use Aperture and never zoom in more than 100%. At 400% photos becomes heavily pixilated (if that is the right terminology) in any case certainly not sharp.
You can sharpen in pp depending on how photos are coming out of your camera. In some cases there may not be much scope to sharpen - depends on the camera and how it has been set up. You can tell when you have overdone sharpening in pp.
My test for sharpness is how a photo looks in the final product - on my computer screen or in a print.