I'll just add something to what eaglesaquarium posted, to do with your method of collecting.
Use a tub or similar container(s) out in the open. Do not use runoff water from a roof. While the plastic roof may avoid the chemicals associated with other types of roofing, it will still be a source of other contaminants like bird droppings, and excrement left by other critters like squirrels, raccoon, rats, etc. Collecting water in smaller containers reduces this, though you still need to be careful. Placing the tub in the open, rather than under any trees, etc, helps.
Some assume that because rain fills natural streams in which fish live, and bird and animal excrement must enter these, it doesn't matter, but it does. Within the artificial confines of any aquarium, even minute additives are magnified way beyond what they would be in nature where they would not even be noticed. So you need to take care.
Rainwater is an ideal water, if it is free of pollution, and obviously suited for soft water fish. I would use this myself, living in a temperate rainforest of sorts, if it were not for the fact that collecting sufficient water for my room of 8 tanks would be a nightmare. As eagles said, having water on hand for water changes, and any emergency changes, is important. But then, my tap water is basically rainwater (our reservoirs are in the Coastal Mountains) with chlorine added, so for all the effort, I would only manage to do away with a conditioner (dechlorinator). There would be other benefits of course; I bet my plants would thrive, the houseplants that only get rainwater certainly do.
Byron.