Rainwater is relatively benign. How you collect it and how it gets channeled to your collection bin/container defines how safe it is for you. If you avoid collection from asphalt roofs, rusty metal trays and gutters, and leaf and debris covered areas you will have the best chance of coming up with clean water. You will want to let it sit covered for a bit to allow for particulates which get caught up to settle out, and make sure that your container is clean and sanitized to start with to make sure there is no growth of coliform or similar harmful bacteria....
Overall, it sounds like it would be much more work and effort that any potential benefits you may receive from use.