I would try to clean in between showers, you will notice that the rain, once dried, will leave rings of dirt on the car, if you use the rainwater you will be rubbing this into the car. I clean my car weekly, its got a great layer of supernatural wax on it as well which just beads the rain off but feel its important to keep it clean even when in rainy season to make sure none of the gunk that falls with the rain settles on the paintwork especially important if you are parking near or under trees etc.