I've got a 7.2 setup and on the few movies it makes a difference, the difference the rear surrounds make is very noticeable. That said, for most movies I watch I've found the difference nice but not very dramatic. If you have room for the speakers then they can make a very nice difference. But in a small room where the couch is up against the wall, it might not be worth it. (btw, If you want to experiment, you can place the rear surrounds on the floor behind the couch firing up. Try to leave a little space between the wall and couch).
Jack