I grew up doing this, though in our case the old heating system drew water from a tank in the roofspace where it could have been left sitting for a long time be

I grew up doing this, though in our case the old heating system drew water from a tank in the roofspace where it could have been left sitting for a long time before being used. We were also taught to run the cold tap until it got really cold (indicating that fresh water from the mains was coming out) before drinking from it. Weirdly, I remember that we had our water pipes replaced with plastic and the heating system modernised when the house was renovated in the late 80s, but we still persisted with those habits afterwards... Australian friends still get super upset with me for wasting so much water - they have water conservation drummed into them from an early age, so tend to see running the tap even for a second or two too long as being some sort of terrible sin.