The idea of rounding assumes that the focal point of a user’s avatar is going to be at or near the center, when that may not be the case.
That’s a good point — I can’t think of a specific example at the moment (edit: duh, Facebook does it), but there are more than a few sites that allow you to zoom in and out and drag your avatar around to specify how it’s getting cropped. Guessing that takes a bit of work, but it would definitely be cool to have (it would even have utility on non-circle avatars).
