Rear surrounds are to be added to an already setup 5.1 system. Dolby PLiix or Pliiz uses information from other speakers when a 7.1 track is not being played. You cant really use the rears in lieu of side surrounds. If you want it to be 5.1 with the surrounds at rear which is not correct placement the you would connect to the surround output on the receiver.
I'm confused. On your first post you said to use the rear. On your next one you said to use the side ( surround ).
I'm only using one set of speakers for my rear/side 5.1 I guess you call it. I put them on my side wall but close to the back wall. I didn't want them on the back as I only have a half back wall and my couch I would be directly under the rear speakers ( about 3 feet above my head ) so I chose to put them on the side walls which placed them about 2 feet from the rear wall.