The "surrounds" should be mounted at ear level or slightly higher, and at about 90 to 110 degrees. For a 7.x system, speakers 6 & 7 should be mounted behind the main listening position again at about ear level. These are Dobly Labs' recommendations not to mention most others.
The problem that comes up often is that specific room layouts do not allow for the recommended solutions, then the "Rules Change" as required.