What I meant was that a "petcock" is vacuum operated. If it's not vacuum operated it has another name. I agree that if it's the old type, that was just a manual switch, then yes, I always used to switch off whenever I turned the engine off. I always got into the habit. But, offhand, I didn't know that there were any modern bikes that still had the old type. Even the geezer, which is just about the cheapest bike on the market, has a vacuum petcock, so it seems that the instructor is years out of date, or just plain & simply doesn't know what he's talking about. Perhaps you could ask him to name any modern bike that uses the old system, to see how extensive his knowledge is.
