Hi Jimmyd,
Using a 12-gauge speaker cable to your sub won't lower the resistance presented by the subwoofer to the amplifier. The impedance of a speaker is largely determined by the voice-coil windings of the driver in the sub, and the crossover, if you decide to build one or buy a simple analog crossover.
Of course, it's always good to use a 12-gauge speaker cable so you don't raise the resistance (and waste power) presented to the amplifier. For short runs, you could even use thinner cable like 14-gauge or 16-gauge, which would not significantly increase resistance.
Regards,
Alan
