I voted never, but that is based on my situation.
Extremely hard water in this area. So hard it eats the anode rods in water heaters within a year or two and gives an odd smell to the water of sulfur. Common practice by plumbers to remove rod whenever possible and run without. Add to that the minerals collected in the hot water heater from the heating process in a hard water environment and that's two strikes.
Since the well water is so hard, we have a water softener. This is hooked up to my hot and cold water except I have a non-softened cold run to the kitchen sink and my brew sink. Brewing with hot softened (salted) water doesn't sound like a great idea to me.
So I use cold and never hot. YMMV but I find that my cold (58*F) water warms up within 10 minutes on the burner.