When I used to do watercolor I almost always matted in white.  It brought out the highlights of the image.  I'm finding that most people prefer black for internet art.  

I wondered about this and a friend explained that monitors 'emit' light so lighter areas seem to glow.  On paper, the lighter parts can only 'reflect' light.  If you print out your work you may need to adjust the gamma a bit to compensate.

Old habits die hard and I still prefer lighter sites and posting to lighter backgrounds.  I'm in the minority though it seems.  Guess I have to rethink it, lol.