Hi terzaghi,
I think you will be OK so long as the center channel speaker isn't more than 2 feet away from the screen. In my experiments, I found that if you move a center farther away than about 2 feet (that's above or below, not behind), then a kind of disconnect occurs, where the dialog seems disembodied from the actors on the screen.
You can mitigate these effects somewhat by angling the center towards your listening area, but 2 feet is a good basic rule.
Putting the center behind the set, if it's at least level with the top of the screen or a bit above, doesn't usually produce a disconnect if you set the distance in your AV receiver properly. In some setups, it may actually improve the depth of the soundstage, especially for music playback.
Regards,
