In the developed countries it is safe to drink tap water directly. Although the tap water is uncooked it has been processed so that it is considered hygienic, sterile and germ-free. On the other hand, the situation in the developing countries is completely different. The tap water needs to be boiled before being consumed.
Let me propose an idea.(For those living in or visiting developing countries) Please use boiled water also for brushing your teeth. Why? Every time you brush your teeth, droplets of the water you are using is retained in the mouth (of course just trace amounts), and some other droplets are unknowingly swallowed. Obviously, boiled water is safer.