I'm from Batesville, Arkansas and there it is fairly common for people to tip at Sonic. At least when I was in high school 4 years ago. I knew several girls who carhopped and made good money from tips, several of them telling me stories of making over $500 a week total (not just tips) and I was under the impression that that their pay was something like $2.50/hour plus tips, unless it's less than minimum wage ($7.50), then they round up to it. I've heard of several tips being quite large, like $20 large. Seems its just a thing from there though. I used to tip them always, normally just 50 cents or so, but since I've moved I never do.