If you run no high-end motors and/or high-quality packs you won't notice the difference.
But for the people who look for the best possible setup, 12AWG cabling is better than 14AWG, but the gain won't be hudge.
But why spend a lot of money on great motors, very expensive ESC's with very low resistance, pushed and matched racing packs and then let the wire eat up the advantage ?
I agree, with the length of the cable, the difference between 12 and 14AWG is not big at all.
But I think you never should save on those (very inexpensive) parts on a car.
Again, there is very good flexible cable out there, which can be well molded into your car.