Is that a bit like when you run a cold tap [and I don't know if this is just my house] but you need to run it for a bit to clear the water that was sitting in the pipes [mine gets quite warm so isn't that pleasant to drink so it needs to be run to get fully cold]? With the hot water tap thing I guess you don't do that so you just get the manky pipe water. As I write this I'll admit it does sound very old wives' tale-y.