Back surround speakers in a 7.1 setup should definitely not be in front of the couch or overhead. In order to create a back sound field they should be behind the listening position, say by at least four feet, and separated widely enough so that both a left back and right back effect would be possible for listeners.
The present 5.1 side surrounds should then be placed directly to the sides of the listening position, since they won't need to help create a "phantom" back effect when actual back surround speakers are in use.
