AJ, although you've referred to these as "rear" surrounds, I suspect that you're talking about what are often called "side" surrounds(i.e. the 4th and 5th speakers in a 5.1 format). If so, you should position them so as to come as close as possible to the usual recommendation for sides, i.e about opposite the listening position and about 3 feet above ear level. This would mean the sloping part of the ceiling about a foot from the back wall and a foot or two in from the side wall(unless the room is very wide,then further from the side walls). The interesting question is whether the tweeters should be oriented front and back or side to side. I suspect that the former might be a bit better, but if possible try a temporary set up each way to see if it makes any difference.
