From everything I remember from my school days and - admittedly - limited research, rainwater is generally not safe to drink as it's washing out particles from the air. While this has decreased in recent time (sulfuric acid in rainwater has decreased by 60% in the last two decades(!) in Germany from what I've found), it's still not near tap-water quality. Unless your stomach is used to this, you really shouldn't drink it if you have an alternative. What really made me understand what this means was when I was in a hotel in Uzbekistan and drank some tap-water without thinking. You don't want to know the rest of the story... I also pointed out that a lot of pollutants will be out of the air during the apocolypse.