Great discussion! The mods update this ancient topic since 1971 when the problem was first noticed. NOTE: to open-track a Pantera in the U.S, one STILL needs TWO MANUAL shut-off valves- one for each heater hose; electric shut-off valve(s) will not pass Tech. This is because the Tech Inspector usually has no motivation or authority to trace out your added wiring, verify the valves and switch work, climb under your car to check hose routings, etc. The second manual valve is so the Tech Inspector is sure the water flow is completely shut off without leaving the core pressurized. Again, he is usually not motivated to have you pull the upholstery panel, trace out the hose routing and verify your single shut-off valve is actually in the pressure line, not in the return- which would leave the stuff under the dash at system pressure.
The second reason for adding shut-off valves is, of course for your personal safety: to keep from bursting a heater hose under the dash at high rpms. This has scalded & scarred the ankles & lower legs of many drivers and a few passengers, while instantly fogging up the windshield- usually at high speed. In updating Pantera water hoses, the short heater hoses under the dash are consistently forgotten, and they do age dangerously.
On the negative side, shutting off water flow thru the heater core for a decade or so sometimes causes a chemical reaction between anti-freeze and the copper heater core, such that when an old valve is finally turned on for some reason, either the heater is plugged, or awful brown goo oozes out of the unused lines and if not noticed, heads for your engine.... A GOOD idea is to cycle your shut-off valves now & then to flush the system. And shutting off hot water to the heater also shuts off the defrosters, which are necessary now 'n then even in sunny California or Florida.