I don't have a window in my current bathroom (sigh), but I know that when the bathroom door is open behind me, I have a hard time seeing myself in enough detail to do hair or makeup or whatever -- there's too much light from behind. So it's a definite real issue, although it might or might not be worth designing around -- a good blind should solve the problem too.

We have a large window opposite our vanity mirrors and have not had an issue, but perhaps it depends on how much light you have around the mirror. We have sconces on either side of each mirror, and then recessed lights above where you stand in front of the sink. We also have a pull up blind in the window...were you planning any sort of window treatment for privacy?