I'm assuming hot water doesn't have copper in it like my mum told me when I was a kid as to why not to drink it. Although if your boiler is a tank based kind, couldnt it be oretty stagnant? Also it warms up your freezer more causing it to use more power?
Only with very old cylinders, it wasn't so much the copper (the pipes for cold are copper too...) it was the fact that the water stored could get all manner of filth in it.
Used to be a certain kind of tank that fed both central heating and taps, I can't remember the name of it now. But those were really bad for it.
I just use warm/ cold water from the kettle myself, preboiled water melts slower.
