7.1 setup

I think dipole surrounds should go the way of the dinosaur. The out-of-phase thing is a holdover from the old analog Dolby ProLogic days when the surround signal was mono and you wanted a vague, phasey surround environment to compensate. Ever since digital surround, more and more surround tracks have pinpoint placement, and with high-rez surround and additional surround channels (e.g., 7.1), the placement of surround directionality is even more specific.

As to the question, although I'd take bipolar over dipole surrounds, I'd say the best is omnidirectional or hemispherical. I've had bipolar and have switched to hemispherical surrounds and it's MUCH better. I use a pair of Mirage Omnipolar RS-1's for the surrounds and a pair of Mirage Omnisats for the rear surrounds. This creates a seamless, enveloping surround soundfield, but if the soundtrack has direction-specific material, you hear it coming and going exactly as intended. An example is the Quidditch games in the Harry Potter movies. On my rig you can hear the broomsticks flying up from behind you and zooming over your head to the front.

No they dont but many do, some even just throw them out of phase by reverse wireing them to get that hard to place effect.

Great questions thegoldenear...
The set up for a good surround sound system is complex for beginners, but if you're asking such questions you must not be (a beginner).
I don't believe that the side speakers have to be bipolar. The essential ingredient in setting up a truely great 7.1 is in the processor, assuming of course that everything is correctly done in the room. (Vague but true).
The set up is everything, and requires a lot of thought, and I would be happy to share my humble thoughts with you if you'd like, as the space here is somewhat limiting.

Larry