One clear place to draw the line seems to be when a customer is not waiting his turn; as a customer, it really annoys me to see someone barge ahead of other people. On the other hand, maybe a customer's annoyance is a signal that the store needs more clerks working at that time.

It sounds like a lot of customers are exercising privilege to a disgusting degree. Or maybe it's the old story of boss yells at man, man yells at wife, wife yells at son, son yells at dog. I'm not sure where a customer service rep falls in there, but it's definitely not boss.