at least in the states, you can tip really anyone that works hourly/commission.
that does include baristas, plumbers, casheirs, taxi drivers, but especially waiters, and bartenders.
we servers and bartenders make in between 2-3 bucks an hour.( if i go overtime i make a whopping 3.50 an hour.) and usually we make our paychecks count towards taxes. in other words, we make jack shit when you dont tip. so if you go to restaurant, thinking that we make anything substantial, or even close to minimum wage, think again. tiping is the way to go, an errbody should do it.
its also a great way to be remembered. and good tippers get special service.