A bipole center rear in a 6.1 setup should work better than a monopole, due to the rear-reversal phenomenon (I think thats what its called, someone can correct
) where the sound from a mono rear center could sound like it came from the front of the soundstage in some circumstances, this is part of why they moved to the 7.1 setup.
BTW, I have bipoles for all 4 surrounds and am happy with the sound field they create.
cheers