When possible, I try to use the same speaker for all 5, although most would say that's only necessary for the front 3. Since most centers are monopole, I guess the question is whether you want to use a dipole or bipole for the surrounds. I think that THX recommends one of the latter. Since many surrounds speakers have a simple switch to change from dipole to bipole, I don't think you have to commit to 1 or the other. I imagine that the diffuse sound of bipole/dipole might be bad for certain types/shapes/sizes of rooms. Purely as a matter of preference, so far I've liked monopole (for more pinpoint sound), although I'm using a Mirage omnipolar at the moment.