I dunno flyboy I started in round motors, and seems to me a shorted mag just might cause an inadvertant start or more like a pop. I was always taught not turn a warm engine to dress the props. Just in case.
Now I've been in the business along time I've learned to never ever say it cant happen.
Regardless of all that. safety is a habit, if you get in the habit of reaching through the prop arc engine off, then one day you will, with the engine running. why? cause its a habit and the glow plug was gonna fall or something else.
I alway teach my students to reach over or around to put the glow driver on. Always approach from the rear to remove a glow driver.