I use 12 guage wire for my fronts and center channel. I use a flat wire that I tacked down to the baseboard (white) that is 16 guage for my QS8s. I then use some really cheap 18 guage wire for the rear surrounds. I have never noticed crackling, break-up, weak signal etc. from the lower guage wires. My runs are all pretty short - less than 20 feet to the rear of the room - so this might play a factor. Also, I've never done a side-by-side comparison to see what I might be missing. But my sincere suspicion is that I am missing nothing, and saved a little bit of money (with which I likely bought some beer, which actually has quanitfiably improved my enjoyment of listening to my speakers) .
Anyway, I see the issue as one of insurance. For longer runs, you will need heavier guage wire. For shorter, maybe not so much. But, many prefer (and are willing to pay for that preference) to have peace of mind, knowing that their guage is adequate.
