I use rain water virtually exclusively, I grow in soil/living potting mix though.
What I can tell you is there are actually other considerations in the process of collecting the water itself, especially if you want the purest/best quality rain water you can get. Were you going to get a rain-barrel or just catch it in a bucket from a downspout? For example you'll want to consider what your roof is made out of, how old it is and how clean your gutters are. Certain roofing materials can impart chemicals in the rain runoff that could potentially be detrimental.
Your roof also collects stuff like leaves, other tree debris, pollen, dust, etc. The more time there is between periods of rain the more crap you'll have built up on your roof and in the gutters. The more surface area, the more room for such crap. For this reason it is actually recommended to buy or build what is referred to as a "first flush diverter". What that does is divert the first x gallons of rainfall so it ends up somewhere besides your rain-barrel. You can find plans for first flush diverters and rain-water filters online just do a search.