Rainwater is not recommendable for a couple of reasons. I've looked into this for my discus, but eventually went off the idea. If you want soft water you're better off getting an RO unit.
The problem with rainwater, in my opinon, is multifaceted. Often rainwater comes from a gutter system (meaning your eavestrough) and there are various sources of contaminents on your roof. Birds often defecate on rooftops and this can lead to some serious pathogens getting into your aquarium water. There are other pollutants in the water as well. Soil, plant matter, pesticides, insecticides, insects, bacteria, algae spores, and sometimes radioactive materials washed out of the atmosphere. You'd have to run it through a complex filtering process, then boil it, for it to be safe. If it's not safe for you to drink, I wouldn't give it to my fish.