Me, I picked up the 'sir' and 'ma'am' after working in a drugstore between sophomore and junior years at university. It's stood me in very good stead. On the few occasions when I'm mistaken for something other than Canadian, it's almost inevitably a Southerner, and when that happens it is always because of the 'sir' and 'ma'am'. (Well, that and because when I get tired I am not always careful about my tendency to match the speech pattern or accent of the person I am talking to.)