For 5.1, use the surround channels. The surround back channels are for 7.1.
7.1 starts making sense when you have a larger room, or when people are sitting off to the sides of the room, or when for whatever reason your surrounds are close to 90 degrees or slightly more.
If it's just you watching movies, or usually just you, then 5.1 is enough.
(I added a rear centre speaker and went 6.1 and I quite like it -- but that's because my living room has an odd configuration and I couldn't get the surrounds very far back (maybe 92 or 93 degrees), so they became easily localizable. A rear centre reduced that problem considerably.)
cheers,
supervij