rain water is generally good stuff, as long as you dont get acid rain in your area. also you need a good way to collect and store it. I think some people use their gutters,,, but If you have copper trim on your roof or copper gutters then thats a nono. Personally I wouldent use rainwater from a roof because I have no idea what stuff is used to make shingles, tar, asphalt etc...
If I were to use rainwater, Id set up a barrel to collect it, just beware that sitting water is mosquito heaven. I feel safer with declorinated tap, or RO better still....