Typical advice about speaker placement is to have them at ear height, pointing at you. This works fine for the front soundstage, where articulation is more important than envelopment. But ear-height placement can be a problem with a 7.1 set-up that has the side speakers directly in line with a row of listeners. For example: if you're sitting in the middle of the couch, then the head of the listener on either side of you will block direct sound from the side speakers. Raising them up a couple of feet (the speakers, not the listeners) will allow everyone to be in clear line-of-sight of the speakers, especially the tweeters.
The height of the rear speakers is not as critical because our human hearing isn't very good behind us (the parts of the ear that help with height cues are pointing in the opposite direction). For the sake of consistency, you can place the rears at the same height as the sides. But if room logistics or some other reason force you to place the rear speakers higher or lower than the sides, don't sweat it: you won't notice the difference.
The other thing about placing the surrounds above ear level is that it adds a little height element, allowing for some phantom overhead imaging. Makes the surround field sound less like a two dimensional ring at the listener's plane and sound more like a three dimensional bubble around you.