I think this depends on a number of things. First is whether your class rank number is reported on your deans letter (MSPE, part of the application to residency) or whether what's reported is a "summary adjective" that reflects your performance relative to the other students. In the case of a summary adjective, it obviously helps to be within that top quintile or so that gets the best adjective. If your school does report class rank, I don't know what you can do besides strive to get as close to the top as possible. Doing the things that will achieve that goal are good in general: acing your tests, honoring your 3rd year rotations.
Perhaps a more important reason to try to get near the top of your class is AOA, which can't hurt your ortho app.
