Some people like a variable phase dial. I find having a 0°/180° polarity switch covers enough ground. Having the drivers in sync is mandatory.
I agree with SirQuack...In most situations drivers facing the same direction and on the same plane, 9 times out of 10 you'll find 0° phase your best option (all drivers in the system are pushing and pulling in unison). However, there are situations (I believe my drivers are wired backwards or the amplifier in the sub inverts polarity) where even room acoustics can create problems. Trust your ears. If your ears do not hear any difference leave it at 0° and don't sweat it.
