The traditional recommendation is dipole sides and direct-radiating rears, at least for those spaces that are large enough to accommodate a 7.1 system effectively. I have the opposite arrangement, but it is due to room constraints rather than choice (my dipoles wouldn't fit anywhere on the side walls when we moved, so I moved them to rears and found direct-radiating speakers to use as side surrounds). I've had decent luck in my case, but I also have really good dipoles spaced out along a very wide rear wall (the room is 23' wide and 17' deep). In spite of what I've done, I would say that the most typical approaches are direct-radiating all the way around or dipole sides and direct-radiating rears, with the latter being the place to start in most cases.