A great post, so many are so focused on saving any way they can and go for what they can get cheap but don't think of saying a little thanks or giving a tip as you say and its something I just don't understand.
A 20 to say thanks that aren't needed is just the right thing to do for me, its all about showing gratitude and respect to people,

How cool was that guy who checked you knew you had given him a $20.

Even if you cant afford to give them a cold drink or some cash people will appreciate if you take the time to say thanks and interact a little with them