I don't really know, but from my limited experience I would say (a) it really helps to mount them at least a foot or two above ear level, and (b) past that it doesn't seem to matter too much. The main thing is that you want some "up" in your "sidewards"

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8