110 degrees. Placed at least a significant distance away. They should be the least noticeable audio-wise, and should not take your attn away from the front. If they are too close, its easier for distraction to occur.
I think the utter ideal would be to have them at the same distance as your fronts, but who the heck succeeds with that.
If you said 7.1 instead, I would recommend 90 deg placement of surrounds for complete wrap. 110 is recommended for 5.1, because the surrounds are better placed straddling side + rear areas.
Ear height is actually ideal, but almost always are the surrounds higher. Why? To fire over blocking furniture, seat backs, and heads.