I prefer my unit frames to be positioned 1/3rd of the way up from the bottom.
My eyes are able to adequately focus on my unit frame, my target's unit frame, my character's position, and the overall events going on around my character.
You want something that is easily recognizable by your peripheral vision, should you have to avert your focus.
Here is a reference, if it helps at all.
http://i3.minus.com/iXthkRsn6AnSs.jpg