Don't forget the inverse square law of light, either. By the time the light travels from the bottom corner of a door all the way to your easel\trays\whatever, it has lost a LOT of intensity. Of course, there is no reason not to apply some weatherstripping where appropriate. But don't worry too much about those tiny little leaks.
BTW, another source of leaks, and techies may want to jump in here, is flourescent bulbs in the darkroom. I had a friend who had them and when he shut turned them off, we sat in the dark for 5 minutes until our eyes adjusted and it sure looked like they were giving off a distinct glow. I know these things don't just shut off like a filament bulb, but anyone know the details on these in the darkroom? I had him remove them permanently just to be 'safe'.