I think it goes without saying that you should wire for 7.1 sound (two rear speakers). I think, also, you might consider (while the walls are open), wiring up two front "height" channels as well. Who knows where the next extension/innovation might be, but it certainly looks like, if 9.1 is going to happen, that's where it will be.
In re: 5.1 vs. 7.1 - here's my take:
I have a 20 X 13 room partially open to one side in which my HT resides. I have M60s and a VP100 up front, QS8s to the sides at 90 degrees to the listening position and two polk direct firing rear speakers about 4 feet behind the seating position placed on the ceiling, angled down.
With a wall about 6 feet behind the listening position (that includes an open staircase), I find that the difference between 5.1 and 7.1 is negligible at most. The QS8s do such a great job of filling up that rear space that, when the PLIIx processor kicks in (on 5.1 source material, including DTS-MA or TrueHD), the sound space in the rear isn't changed that much - no "new" sounds are suddenly behind you. Rather, the sound is better reinforced (not necessarily louder, just more specifically placed) at times. Toggling between 5.1 and 7.1 the difference can be detected, but after a couple of minutes of listening, the effect is approximately the same in both 5.1 and 7.1 environments.
