Etiquetteer has a problem with "No problem." Especially as a response to "Thank you!" and most especially when uttered by customer service personnel. Why? Because the idea that serving a customer might create problems (even on the occasions when they do) is not something that should be communicated to the customer. When Etiquetteer thanks the waiter for serving dinner and hears "No problem!" back, Etiquetteer thinks "Well I hope not. That's your job!" A courteous, smiling "You're welcome" leaves a Much Better Impression.