In cases of low power tranfer like the power to your door speakers the current travels with relative ease along the 18 to 16 gauge wires so a noticable difference is non existent.
In the case of high power subwoofers though where you have a pair getting beyond 500 watts rms each it is a good idea to use a beefy speaker cable of equal lenght to each sub, something like a 12 gauge will work well.
I personnally have seen two subs moving at slightly different times from the same mono channel amp in the same enclosure because of the speaker wires being a 16 gauge of longer length and the other was a 12 guage of proper length, you could actually see the subs moving at different times while playing lower test tones. This problem was corrected by replacing the long small cable with an identical to the other sub.
In the case of high power subwoofers though where you have a pair getting beyond 500 watts rms each it is a good idea to use a beefy speaker cable of equal lenght to each sub, something like a 12 gauge will work well.
I personnally have seen two subs moving at slightly different times from the same mono channel amp in the same enclosure because of the speaker wires being a 16 gauge of longer length and the other was a 12 guage of proper length, you could actually see the subs moving at different times while playing lower test tones. This problem was corrected by replacing the long small cable with an identical to the other sub.
