Phone reps, of course, have the difficulty of not being in physical contact with you. The others, though, my grandparents actually would tip house cleaners. Would get annoyed if we left out cash in the hotel, as "you should only do that if you are wanting the help staff to have it. And then, you should do it in an obvious way so that they don't feel like they are stealing." I distinctly remember them writing notes and leaving them with a tip on the desk.

Well, one does handle your food before you eat it - and before you’ve seen it.