I have a 5.2 system in my office, haven't set up the big HT system yet, but I ran a 7.2 system in my last place. When I set up the big system again, I think I'll leave it at 5.2. In any case, I don't think there is any 7.1 software, (movies or music) anyway, I don't think the back surrounds add much if your side surrounds are well placed. Bottom line, IMnsHO, if you want to go with a back surround, go with a single center, since the back channels are mono anyway.

Enjoy the Music. Trust your ears. Laugh at Folks Who Claim to Know it All.