water heater hot water line shut off valve

i am about to replace the gas hot water heater. it currently has no shut off on hot water side. any reason not to add a valve to hot water side?

Let's say the water is cold, the inlet and outlet shutoffs are closed, then the WH is turned on. The water expands, and if everything is working properly, the T&P valve will open to relieve the pressure from the hot, expanding water. No different from what would happen if you don't have an expansion tank, and all of your valves are working properly. No big reason to add one, but I suppose it won't hurt; less water to flow down and out to drain and then be able to solder the new connections. Don't know all of the codes, but one of the pros will chime in if it's an issue.

I've asked this question before. There is nothing wrong with doing it. One on the cold side is required, another on the hot side is OK.

Unless you are in Chicago. It is against code there.

From the 07 Chicago plumbing code:
18-29-503.1.1 Shut-off valves Shutoff valves for water heaters or heated water storage tanks may be installed on the inlet side only.

Most people just simply don't want to pay for another $17 valve and the time that is needed to install it when you don't NEED one.

But in a house you really don't need one (unless you have a large house!) because there isn't that much water to be drained back and you are only affecting one person, yourself!

In an apartment of multi-dwelling residence I would certainly install one because it would be pain in the butt to go around to all the units and get them to open all their fixtures up to hot to drain off all the water. Without one you could be there an hour or more just waiting for all the water to drain back!!!! What a pain.