Ear level is “technically” better, but you can try all positions, and see what works for you best. After all, it’s your ears that need to be satisfied, not ours. 

A practical consideration is the relative distance between a listener and and a rear speaker. A not so great situation is if the surrounds are placed on coffee tables immediately adjacent to each end of a sofa. A listener in mid sofa will hear balanced sound, while one or the other speaker will dominate for a listener sitting at either end. Even if the speakers are no longer at ear level, a more distant mounting scheme will minimize this issue.

Aiming the speakers at the listener usually has benefits. Some speakers significantly beam the highs. ONE’s have a relatively uniform radiation pattern. You can explore the significance of beaming (for you) by setting up the ONE’s as a stereo pair or simply use a single ONE, then listen at various positions.