We normally stick with a single rear channel surround speaker unless we are using THX Ultra 7.1 processing (or similar true 7.1 channel mode like 7 Logic). The reason is that if you have two speakers playing the exact same signal, you get frequency responce problems, i.e. notch filtering.

I would put the side speakers 6' high and slightly behind the main row of seats (good for DVD-A & SACD) and a single rear speaker 6' high and in the center of the rear wall. The rear center would not be active for DVD-A and SACD.

I would prewire for 7.1 in case you upgrade to a THX Ultra processor. If your surround speakers have smoother frequence responce on axis then you could angle them down a bit.

Good Luck!