According to Stormin Mormon :
If you shut off the water main, and then open both faucets on the laundry
sink in the cellar, there won't be any pressure in the system. So, you won't
have to shut off the water supply to the water heater. D'uh.

It's a good idea to shut off the water supply to the water heater because
trying to drain the lines while working on the supply end could end up
having to drain the whole HWT. Turning off the HWT water ensures you
don't have to refill it again.

Unless you meant something different like shut off the electric or natural
gas source of heat.

When working on the water supply, it's always a good idea to turn off
the water heater. Especially if you're going to drain the tank.

The quickest way to burn out HWT electric elements is to supply power to
it while the elements aren't immersed.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.