This holds true indoors as well. If your garage floor is at or very near grade, a moisture barrier between the concrete and your car will reduce the amount of condensation and subsequent corrosion on the underside.

Free air flow helps as well, so if the car is covered it's best to leave the sides and ends pulled up a bit so that air can flow. Cold air retains very little moisture, it's the temperature swings that bring on the water.

If you find a car that has spent a few years sitting in a grassy lot, the belly will be in deteriorating condition. Rocker panels and floor pans really take a beating from vegetation.