My area of concern lately has been hotel shuttle drivers on airport runs. I give them, generally, a flat $2 per ride, figuring that a cab would cost a lot more and mass transit (I travel rather light) might be cheaper, but more of a pain.
I would prefer that I keep both bags with me and not trouble the driver at all. However, the guy (they've been almost exclusively male)tends to grab the larger (neither heavy nor bulky) bag and put it in the back. Objecting seems silly, but I do wonder whether that it is "justify" some expectation of a tip?

Unless the person is very friendly, offers some insider info on things to see/do, etc. I rarely tip more than a flat buck for a beer or two. To me, working at a counter and plonking a bottle down (or pulling a draft) is not the same as carrying dishes across the room.