I assume that the OP is talking about framing and not focus. The accuracy of the viewfinder of an SLR is determined by the size of the mirror. There are several competing problems here. If the mirror is designed to provide 100% coverage the inertia of the mirror becomes a problem. Camera shake also increases with mirror size. Most SLR's are therefore a compromize and provide less than 100% coverage. Rangefinders with frame lines actually show more than 100% coverage for most lenses. I think that most people do not really think that somewhat less than full coverage to be a problem.