Not critical; the common suggestion is about 3' above ear level. Since they're still "side surrounds" even when mounted on the back wall, they should be separated to the side as far as possible, to within a few inches of the side wall on the left, to give the outside left tweeter a little "bounce room".
Alternative suggestion for 7.1: sitting opposite back edge of fireplace, left surround directly to the side about 1' above fireplace, right surround now has room directly opposite it. Back surrounds about 1' lower and 2' inside of present positions.
