7.1 speaker placement--how should I aim rear bipoles or dipoles?

My HT room is basically a big "U" -- there is no back wall. (Well, there is, but it's another 20+ feet away.)

I'm going to have side and rear surround speakers (HTD Level 3) that are switchable between bipole and dipole. Aiming the sides is straightforward, as the speaker can sit flush on the wall.

But what about the rear surrounds? They will still be mounted to the sides of the room, since there is no back wall. Can they be flush to the wall like the sides, or do I need to ceiling-mount them and aim them at the screen?

When I say "aim at," I mean directing the portion in between the drivers--I realize that you are not supposed to aim the drivers themselves at the listeners, it defeats the point of this speaker design.

Thanks for your input!

I could. Nothing has been decided yet. I was hoping to use bipole/dipole surrounds because of how great everyone says the effect is, but maybe it just isn't right for my room, at least for the rears.

Followup--here is a diagram better showing my predicament.

I talked to tech support at HTD and they suggested monopole/bookshelf speakers for the rears. He allowed as bipoles would kind of work though you'd be wasting half the output by firing it into the back of the room.