First, this is a topic of debate, so if you are looking for a definitive conclusion....sorry!
For film, higher up would by optimum.
For MC music where the rears are just ambience, higher up would also be a good choice.
But if you like MC music where there are instruments placed in the rear channels, then for me, ear height is the way to go (I mount the rears a bit behind and to the sides of the seating position).
I was OK with high mounted surrounds until the day I bought my first MC music disc with discrete instruments panned to the rears (Steely Dan - 2 Against Nature). Having saxophones and vocals coming at me from up high just did not work for me.
You can argue that having instrumants behind is phoney to start with (point taken), but for me, behind and up high was just too phoney.
Its been my experience that a rig set up as I currently have mine works OK for films, while being superior for MC music.