In a 7.1 setup the side surrounds should be directly to the side of the listening position, since they don't need to help supply a rear sound field. The back surrounds should generally be spread about as far as they're behind the listening position, but also at least enough so that the listeners have one behind them to their left and one to their right(i.e., wider than the width of the listening position).

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Enjoy the music, not the equipment.