My uncle is a plumber and scolded me once when he saw me doing it when I was younger. It's best not to. The hot water in your home has been sitting in your hot water tank and can have unsafe contents like metals, and a ton of bacteria. Over time your hot water tank needs to be replaced because of corrosion and the bacteria that collects inside it - where your water is. It's better to use cold water, even if you're boiling it. It's unlikely to kill you or anything if you do drink hot water from the tap, it's just best not to. If you haven't used the tap in awhile (over night) it's also a good idea to let it run a second or two before you use the cold water.