Nice video Sab, you appear a very cool calm operator! Wish we had your weather right now. As you've probably guessed, turning the taps off here in Blighty (much like adding milk after the tea) is considered a bit left field - probably to the relief of the AAIB.
What's the background to the instructor choosing this method of gliding selection, i.e., does the type you fly have a history of fuel cocks vibrating closed? It didn't look like you were too surprised by it and the instructor mentioned 'everyone expects it now' which to my mind negates the exercise.
To answer your question, I've only ever had the throttle chopped on me during check rides to drop the field landing hint, no-one has been brave enough to turn things off. Mind you, if you flew with me you'd want to leave things switched on frankly!