I would suggest going with Dolby's guidelines and mounting the L/R surrounds to directly to the sides of your listening position -- especially if you have (or are going to have) Axioms QS8 or QS4 surround speakers.

Most discrete surround effects in movies are not meant to be localized. If you place all your surround speakers behind you, that would ruin the effect, as you'd redily be able to hear all surround effects coming from behind you.