FWIW if the conditions are such that the autofocus can work, I find that it is almost always better than what I can do manually. Indeed, I watch the focus indicator in the viewfinder as much as the screen.
Remember too that the this series of nikons has a focus mode where it will shoot a frame whether it's focused or not. (The basis is that sometimes a blurry pic is better than no pic)
Finally: There is a corrective lens on the finder itself for adjusting for people who are near/far sighted. If this is set incorrectly, you will have a harder time judging the focus in the viewfinder.