There are no "rear" seakers in a 5.1 setup, you actually have "side" surrounds. If you go 7.1 that is when you would introduce a rear pair.
It would make good sense to use the terms this way, but I see "rears" often used to refer to the surrounds of a 5.1 system. Since the surrounds of 5.1 correspond to the side surrounds of a 7.1 system rather than the rear or back surrounds of a 7.1 system, the term "rears" is ambiguous, unfortunately, unless you know whether a 5.1 or a 7.1 system is under discussion.
