Dipole speakers in a 7.1 setup?

I have a mixed and matched M&K 7.1 speaker system consisting of 4 S85's, one S85-C center, one pair of 500THX dipole surrounds, and v-75MKII subwoofer. My question is, in a 7.1 setup, would it be better to use the one pair of dipole surrounds I have for the rear side speakers, or the rear back speakers? Thanks very much!