Anywhere you would normally tip in normal day to day life, you tip while on tour. Many service workers depend on tips as a major part of their income. Personally if I was a waiter and you didn't tip I would certainly say something out loud and embarass you. There really is no excuse not to tip. It should be considered as part of the price of what you are buying. Coffee, yes. Grocery bagger, yes if there is a tip jar, then it is expected. Restaurants or diners, in the US, always.