Marc, I would recommend different layouts for the side-surround speakers, depending on if its 5.1 or 7.1.
If its 5.1, I'd go for about 110 degree angle, to have them straddle the sides + rear areas for best balance/coverage.
In a 7.1, I would put the side-surrounds at 90 degrees for best wrap around and side vs rear imaging.
just an fyi.