I don't think gratuity for cab drivers can be looked at the same way as it is for wait staff at a restaurant. The companies I have worked for, past and present, have a lot of poor, regular customers who go back and forth to work and cannot afford a car. These folks usually want all their change back--even on a $5.80 fare. The tips add up for them, so they don't tip. It doesn't mean that they have any problems with the service they receive. They simply can't afford it. Bottom line: There are no hard, fast rules about tipping cab drivers.