Good question. Depending on the style of the fixture, but I try to mount it with the bulbs pointing down so they don't collect as much dust.

If this is the bathroom that you use for shaving and/or applying makeup it is best to have as much light directly on your face, on all sides if possible, so point the shades down and get a fixture with at least 4 shades for a vanity that big, this light is too small, you want the lights smaller than the vanity and the mirror but not too small....if it is a powder room I like the lights pointing up, it looks dressier