On top of that, it will have to work harder to suck down the gas. Also, your fuel filters will get dirtier and your fuel injectors have a greater chance of getting clogged. Throughout the lifespan of a car, little particles of dirt may find their way into your fuel tank, and if you run well into the reserve tank, the dirt sits at the bottom, and you risk sucking the dirt into the system.
I do this shit all the time. I can drive to work and back with the light on and that it about 26 miles total.