I have found that even in heavy rain, not much dirt is added on to the car if it is left outside. If it dries without me driving it first, I take a detail spray to wipe it off and little dirt comes off- but the car looks better without the spots. However even if I don't wipe it off for several days the rain spots easily come off unlike sprinklers.
My car was even outside last year during two hurricanes, and it still looks brand new. The only thing I don't like about rain, is driving on a wet road because then the car gets all the dirt sprayed on from the road
Bottom line is don't worry much about rain water in itself- it is not a biggie- unless you live in an area where the water is full of pollutants etc.