I think that the more sites the better. Having competition will drive prices down, to try and attract the buyers, but that leaves the artists out of the pictures. For us, that means that by offering decent prices for our members, we are guarenteeing a future as well as quality for our clients.
It is perfectly fine to advertise your images through dozens of sites, but make sure you are able to keep the licence for them. They are your work to begin with.