I've worked as a cashier. Once, a man came through my line talking on his cell phone, and he had thrown down a check on the counter behind his items. I needed more information for the check. For several minutes, I simply stood quietly, but he wouldn't even look at me. Finally, I simply said, "Sir?" to get his attention. He was extremely angry and told me to shut up because he was talking.

While anyone working with customers needs to be polite, the cashier deserves the same respect.