In my opinion upload your books to every site necessary or at least to ones more relevant. Amazon is what brings more exposure to the book, maybe more sales but Smashwords has many benefices and people will prefer buy your books through there. It's the indies paradise.
Another e-bookstore you may want to consider is
Feedbooks, don't have many experience with it but people that use it seem to be satisfied with the service. And they have some partners too.