If you are growing in good healthy soil, the rain water should be good for your plants. One would still need to be watchful for the signs of nutrient deficiencies and adjust or augment the feeding schedule appropriately. I would not recommend using rain water in a hydro set up because the air contains almost every known bacteria, mold spores, funguses, pollen, small insects, and all sorts of things (not to be mentioned in good company (lol)). Using rain water in a hydroponic set up could give you more problems than benefits.