It is generally safe, although there is potential for it to contain particulate and pollutants, depending on the area where you live, how long it has been since it rained, what the wind is doing, etc. When there is pollution in the air, a rainstorm essentially will wash it out of the air and deposit it on the ground. There are people who collect rainwater and use it in their tanks. I personally never have, and live in an urban enough area that I wouldn't do it.