I tend to tip in international locations as many have either free drinks or a cocktail hour where they take orders and serve you. When the staff remembers my drink the next day (Singapore, Bangkok, and some others) it really shows what service is about and always wants me to show thanks with a tip.

In the US -- sometimes out of pitty in a slow lounge where they have been friendly, or as others have said... provide some additional added value.

I would tend to leave more at breakfast than if just stopping in for an afternoon / evening snack since I make a bigger mess.