I was always brought up that 15% tip was a standard rate. So it was surprising when my mom argued with me today that it is now considered rude to tip 15% and that 20% is now the standard tip. This is after I expressed surprise at her tipping over 20% for mediocre service from a delivery man. I have nothing against tipping, but to me, 15% is for average service, higher or lower for better or worse service. Since tipping goes by percentage I don't see any logic behind this supposed change. Being a college student I might be out of the loop. Thoughts?