Maximum Surround Placement Height

Can anyone offer some guidelines as to how high is too high to place direct radiating surrounds? I am in a position where I will probably have to place them about 8-9 feet high. The room is about 20x20 with vaulted ceilings. We are building a house and I have the opportunity to have the wires run at almost no expense, so I won't have the opportunity to experiment to see how they sound that high up. Thoughts anyone?
Thanks

When I was placing mine I researched and found that putting the tweeter 36" above the ears when seated was the "recommended" height. I was able to accommodate that in my room.

I would put yours as low as possible. Since 36" would be about 6.5'-7', 8' shouldn't be too bad. Choosing speakers with good off-axis dispersion would also help.