If you have an AV receiver with the ability to decode Dolby TrueHD and/or DTS-HD, and you are playing 7.1 discrete content, then you will want to separate the back surround speakers to produce a 60 degree angle from the main listening location.
In addition, you should make an adjustment to your receiver setting. Simply go to the THX Setup Menu in your THX Certified AV Receiver/Pre-amp and set the ASA Surround Back Speaker setting to "Apart (greater than 48)."
Check out these additional placement tips for your front speakers, center channel, side surrounds, back surrounds and subwoofers.