Besides what Peter has described, the answer is yes, you can use just one speaker in the center back for the 6th channel. The reason that it's often suggested and illustrated that two speakers be used in back although there's only one channel(there's no "real" 7.1 format)is a psychoacoustical phenomenon which can cause a sound meant to be heard directly behind you to instead appear to come from directly in front. This can sometimes occur when only one speaker, directly behind you, is used; it can't happen when two separated speakers are used in back. Another possible reason for two back speakers is that some special processing(e.g. DPLIIx, Logic7)creates a difference in the signals which would go to the left and right back speakers, although this still wouldn't be "real" 7.1.
