I can see Smarthawk's point about the 'off' position maybe having a fault, so by all means put mags to 'off' on shutdown. But as I keep saying the damage can be done when mags are switched off then on again while the engine is still turning.
So if you want to check both mags are 'off' in the 'off' position, turn the key to that. But why turn them on again having proved the engine stops firing?
Civvy Chippies I've flown were always switched off by switching off the mags and fully opening the throttle - there no other method (OK, you could turn the fuel off and wait a few minutes for the engine to stop!). In the engine compartment one could see where there used to be cable coming from the RH side of the panel, but I always understood that to have been to cycle the cartridge starter, which was removed when Chippys were civilianised.