Doesn't matter where you mount the shut-off valve - either intake or outlet of the heater.
If the valve is turned off, there is no flow through the heater hoses either way.
The purpose of this valve makes sense especially for hot summers.
During the hotter months, you want to turn it off, so all the coolant can flow through the radiator for vital cooling.
If you leave the valve on, a portion of hot coolant bypasses the radiator at all times and will just return to the block uncooled.
The small heater core does get hot, however never removes enough heat to cool an engine and certainly doesn't help in keeping the cab cool.
Don't know how hot your summers get, but in the desert you take as much cooling as you can get and a valve can help.
AD trucks had an automatic heater valve that was installed inside the cab on the fire wall.
It measures the internal cab temperature and turns the heater valve on and off automatically ... a 'cool' accessory indeed.
