I don't have all the answers, but see if these help...
The positive terminal on the speaker should be the one which, when a positive voltage is applied, causes the cone to move outward (from the basket or frame). Simply hook a D-cell battery to the speaker terminals and see which way the cone moves... when it moves out the positive terminal of the battery is hooked to the positive terminal of the speaker.
~toaster
The computer sure does like to shift a lot. I'm guessing that may be why that statement was made.
In practice it should not make any difference, so long as both speakers are wired in the same phase... ie both speaker cones move out together in unison. However, if I remember my stuff from the days of HiFi (predates WiFi by many years)
~toaster