Hmm, I'm not sure I agree with that. The rear surrounds are more for ambience and the occasional direct sound. I don't think you want them right in someone's ear. Especially the people sitting on the sides will hear too much of the surround closest to them.
Consider how movie theaters mount their surrounds all up high.
Rear speakers want to be above ear height in order to prevent localization effects. Humans are really good at "hearing down" (as a species, we learned to find most of our threats on the ground, not in the air and thus adapted to that). Surround speakers are not point sources...but ambient sources. Thus, place the speakers accordingly.
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