Most people that put them closer to the ceiling are doing so only because of some other physical limitation. About a foot above seated ear height seems ideal. Now, that is based off of different factors.
THX recommends 2 or more feet above the ears, but they are generally basing that off of direct-radiating surrounds (like putting M2s or something in for the surrounds) where the speaker points directly at the listener. With the QS series speakers, and with nothing firing directly at the listening, you can drop that down a bit without the worry of "blasting" sound right at your ears.
