From ********** "Regardless of exactly where they are placed on either wall, make sure to locate them above ear level or so that they do not directly radiate toward the listener. With surround speakers, some reflection is intended before reaching the listener's ears".
The general concensus seems to be "above ear level", but how much above ear level? The ecoustics link above says "about 2 feet above ear level", but I would think it depends on how far you are from the speaker, right?
The speakers are being wall-mounted so they really can't be moved (without patching up the holes, repainting, etc) after they are installed.