I take it you have not actually seen how rust develops? And why would you paint the underside when it's already painted?The best rust preventative is a good paint. Once you oil it up you can never paint it. Well yeah but the cost to clean ALL the oil off would be very high. New vehicle, make sure it is painted real good, and let the paint cure then oil it if that is what you need or want.
Paint doesn't need to be reapplied every year or so. It just needs to be applied the correct way.