Hey 300, It won't hurt anything to install shut off valves but why do you need to install them? Is the heater not shutting off like it should? Is the heater core leaking? Just curious. You only need to install one valve to stop the flow if it isn't for a leak, if it's the core leaking you will need to install a valve in both hoses.
As an added note, if it's just due to heat in the car, there are many cars that have the vacuum operated shut off valve already in the line just before it goes through the firewall activated by your heat control button or lever, maybe it gave up the ghost. There is a thread that covers this somewhere if you do a search here on the forum.
