In my opinion, you should specialize where the audience is biggest for your kind of art. No point in wasting time with a site where you'll never get traffic, when you could have been focusing more attention on to one where your type's popular.

This is pretty much my stance on it. If you really want to get your name out everywhere, then sure, go ahead and get yourself set up on every site. It's a lot of work to maintain multiple galleries though and some of them always end up lost in the shuffle.