It depends on your definitions: "turning off the engine" can mean simply pulling the mixture for a practice engine failure.
That is a required part of light aircraft training! Glide approaches from base - as the Practice Forced Landing is rarely completed to landing in a paddock, I would have done this with every student.
Turning off the mags (switching off the keys) is a little rarer - removing the keys is stupidity, as it serves no training purpose.
Stopping the prop? I was introduced to this in my aerobatic rating, and it happens as a matter of course in a power off spin in a Robin (at least the one I fly) so it has a positive training value to demonstrate restarts to students who intend to fly aerobatics - both with the starter motor and using a dive start technique.
I wouldn't stop the prop for a glide approach and landing, though, as I cannot see any training benefit over a glide approach with a windmilling prop - and a much larger risk increase, as in the "remove the key" case.