Jason, of course a 7.1 receiver can be used with a 5.1 speaker setup. The receiver will be set to show no back surround speakers and won't send any sound to the back surround terminals. So yes, you can certainly use the side surround speakers without back surrounds. Unless you have a significant distance behind the listening position(say at least 4')for a rear sound field to develop, a 7.1 speaker setup may not be of benefit.

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Enjoy the music, not the equipment.