This observation seems to hold true in most cases: Mother worked as a waitress many years ago. My wife worked as a waitress before we married. Waitress or server are always treated respectfully and tipped 20% and up unless service is crappy. Other members of our extended family never worked as servers and it often showed. One would give a measly tip before the meal and proceed to treat the server as an indentured slave. Others would try their best to make life miserable and leave a paltry tip, if any at all.
Folks working at sub-minimum wage jobs have a tough lot, so I try to show my appreciation their efforts.