If you mean we were always told not to drink from the hot tap at home, that was because of the way it was heated. In the old days hot water was fed from a header tank in the roof which could get dirt or anything in it, then kept warm in a cylinder thing.
With modern boilers which take water from the mains and heat it instantly that doesn't apply.
Safe as in bugs in the water etc. I don't have gas or a boiler, just an immersion (sp?) heater.
I know it's a silly question, I also wondered if warmer water would give them tummy ache as always used to cold