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.
Gotta disagree. If it's on the "pull" side of the pump & turned off the pump is putting pressure on the heater core all the time even when no fluid is moving. If the valve is on the "push" side & turned off the heater core isn't under pressure & has less chance of failure if it's a bit weak.
