Has anyone considered it could be the opposite? If a restaurant is cash-only, or if they don't take American Express (so I have to use Visa/MC), I don't tip as well. Those are punitive factors in my tipping algorithm. Unfair to the wait staff? Perhaps a bit, but I'm tipping on the overall quality of the service, and not taking American Express (or worse, not taking cards at all), is a substantial harm in the quality of the service, and will be reflected in my tip.