That's certainly true with my setup. Zilch from the surround backs for those two movies.
That being said, they've done a great job mixing the surround sides, because there were times I thought for sure the backs were participating in the soundscape, it was that full. However, rewinding the scenes and putting my ear up to the speakers revealed nothing coming out of the backs.
Until someone tells me they're getting a discrete mix from those back surrounds (for episodes V & VI), I'm thinkin' they are mute. Whether that's intentional, is another story.