It depends on what you want them for. If you want a colorful egg basket, then get mixed breeds. Get the chicks at the same time and raise them together, so they get along better. If you want to breed to a standard of perfection (SOP) of a specific breed or to keep a rare breed in existence or if you like a particular breed or egg color over others, that's another story.
I would recommend that if you choose to mix breeds in one flock, try to get breeds that are at least somewhat similar in appearance or physical traits and especially be sure they are hardy in your local climate. For example, choose all clean legged or all feather-legged, because if you mix those, the odd-ball may get picked on for being too weird. Feather-caps like polish may have trouble seeing and get picked on more as well. 
Another thing to consider is, if you mix breeds, then you don't get to see the variety of personalities that can come out even within a single breed flock. And if you decide to breed, the chicks may be barnyard mysteries. Many people are OK with that too. Eggs are eggs!