I built a RepRap 3D printer using Pololu stepper drivers with heat-sinks. Even with the heat-sink, the motors would draw enough current to overheat the driver chips, causing them to go into fail-safe mode... they stopped operating until they'd cooled down. While this was only a second or two at a time, this caused the motors to randomly skip steps, which is a real problem with 3D printing. The heat-sinks, on top of chips barely 1/4" square, got nearly hot enough to burn my finger.
A tiny CPU fan over the controller board solved that problem. So yes... a fan can quite literally make your robot's motors run more smoothly.