Coming from stuff I've been reading:
For home theater: ranges from just high enough to clear heads, to ear level when standing, to 2-3 feet above heads when seated. Experiment! Find what sounds good to you but keep same height for both. Try 6' first. I find that just right for me. The spaciousness doesn't sound too high for me. But that's me. It's your system.
For 5.1 audio: 48 inch reference height for all speakers and up to 15 degrees more upward with reference to seat on rears if desired per
Dolby. See Figure 3.3 for furthur info.