In the US, is it customary to tip the line personnel? The etiquette of interacting with the line personnel wasn't really taught to me during flight training, and I feel that every time I ask for line service, I'm doing something wrong by not tipping. I don't travel with groups or other pilots very often, so I don't see this in practice...

Do line workers in the United States expect tips for service (fuel/parking) from GA pilots?
I've never tipped for aircraft refuelling or other line services, and have never been criticized or looked at the wrong way for not doing so. That said, tipping in the US has nothing to do with the industry in which the person works; it's a personal reward for good service, and it is both appreciated as well as remembered the next time you are there.