You know why I never checked the polarity of my stereo speakers? Because I can't tell the difference.
I was told that as long as the polarity is the same on all speakers then it should sound fine. So if you have the wires going from + on the Amp to - on the speakers then there no problem if all speaker are configured that way.
If you have a mixture of polarities on different speakers then the speakers will be out of phase. To be honest I've never tried setting it up that way just to see if it noticeable.