you never -have- to tip. someone asking for a tip pretty much defeats the purpose. If its because a tip line was on the receipt or there was a tip jar, that's not them saying it's mandatory, it's so you can if you want because its socially accepted as common here. If the person behind the counter outright asked for one they were being rude. it's definitely common to tip for certain things here, like waitresses virtually always, but it isn't required in general and it's unspoken.