I'll second all of Kevin's points, especially the one about front/back reversals. It's an old problem with an incredibly simple solution: use two rear speakers. Note that the surround-back channel is the ONLY CHANNEL where THX and DTS and Dolby all recommend the use of two speakers.

Also note that when those companies refer to their EX/ES technologies, they always use the term "surround-back", never "centre-rear". This is because those sounds were never intended to be localized in the middle of the back wall. To that end, two rear speakers (7.1) get you closer to EX/ES's back-wall-of-sound than a single speaker (6.1). While individual surround speakers are less localizable in a 7.1 set-up, sounds themselves can be better localized when using four surrounds. Sounds intended to come from behind really do appear to come from behind, and not some phantom side-ish rear-ish direction. Fancy processing? Nope, just a couple of speakers behind you (makes it hard for those sounds to come from any other direction).

That sort of stable imaging in the surround field is simply not possible with a 5.1 set-up. Nor can two surround speakers provide the sort of wrap-around envelopment that four speakers can.

Best,
Sanjay