If you do not yet have surround speakers in your system, I would recommend mounting them on the side walls as close to your listening position as possible. Most movies have the left/right side surround channels mixed in. It is intended for sounds directly to the left or right of your listening position.
There are some movies with an encoded 7.1 channel mix included. These will add the back wall surrounds. They are more for ambient sounds. However, the left/right side surround channels are more important.