Humans are wired to derive dispositions, including ethical and niceness-related dispositions, from our past behavior. Not tipping sets a bad internal precedent even if no other person you care about signaling to sees you. It sends a bad signal to yourself and runs the risk of making you a worse/meaner/self-serving person compared to what you are/should be.

Thanks, I think this is the point I was missing. It's a good point.