Twelve gauge copper wire has resistance of 0.00187 Ohm per foot. That means that the 12 foot cable will have 0.026 Ohm more resistance than the 5 foot cable. You probably get more difference than that just from unequal torque on the binding posts for one channel vs. the other. If you use smaller gauge (larger diameter) wire, the difference is even less. Similar arguments can be made for the inductance and capacitance of the wire.
Oh, and in case anybody brings up the idea of propagation delay through the wire; the signal passes through the wire at a significant fraction of the speed of light. Even at 1/10 the speed of light, that extra 7 feet will be traversed in 0.000000071 seconds. I don't think you'll hear any phase difference from that time delay, but if you do you can compensate by moving your head about 1/2000 inch toward the side with the longer cable.