Yeah - that used to be the rule.Im pretty sure idea of warm water sitting in a dark tank for a while stagnant puts me off using the hot tap for anything that involves my mouth.
Don't drink or brush your teeth with water from a storage hot water system because they used to just be fed by open troughs in the roof... which had dead pigeons in them.
These days most people have instant hot water in the form of combi-boilers, so it doesn't really make any difference.
I think if you can remember the days when you weren't supposed to put the water from a hot tap in your mouth, you're showing your age.