Actually... I've heard some theorize it (dents) might but I don't know if they'll create the right types of turbulence reordering/vortex generation you want to clean up the air around you.
Might see bigger gains in FE with more air pressure in the tires, cleaning/smoothing out the front of the car, adding some grille blocks to limit the amount of air that gets caught up in the engine, and maximizing weight reduction. Smooth hubcaps might help too or a rear wheel skirt but one step at a time.
