We had this chat several years ago....you, ubertuber were also involved.
Check out
this thread.
I referred people to this
"speaker wire history" article written by a fella named Roger Russell. He is a speaker inventor and designer retired from McIntosh Sound Labs (arguably one of the finest makers of audio appliances ever). He maintains a pretty interesting
web site, with lots of various audio and speaker information as well as a number of other things.
Anyway, the bottom line is impedence. You'd be hard pressed to create the kinds of impedence which will make any discernable difference, with just about any old guage of copper...BUT when dissimilar metals contact each other....as they do when you crimp the copper wire into your stereo, corrosion happens and this will introduce the unwanted impedance.
Hope these links are helpful.
-bear