How can checking that each mag works independently confirm that they both shut off when the key is turned to off?
Just prior to shutdown you're running close to idle, so the rpm drop when you switch to a single magneto may not be noticeable anyway. You might check R, check L, cut the mixture, turn the key to off, and walk away not knowing that the mags aren't both off.
It's true that if something has gone wrong with that switch, it might show up in the L/R check, but do you want to bet your right arm on it?