Hi Tim...

This being an electrical appliance with water I'd have to say that yes, it is a good idea to unplug the water heater when doing any kind of work on it.

If the relief valve inserts into the top of the water heater then you really only need to shut off the water and drain the water to just below the relief valve and remove the old T&P valve and install the new one, but I wouldn't recommend it unless you have some previous experience with electrical/plumbing, OK?

If the T&P valve inserts into the side then yeah, I'd shut the electrical off and then do the job. I would not plug the unit back in until the T&P valve was replaced, the water heater was filled with water and the air was cleared from the hot water system through the faucets in the house.

Good luck...

Mark

timmshouse, to change the TPR valve, you should turn off power or shut-off the gas to heater, turn off the water, drain the tank to a level below the tpr valve and then make the change. Good luck