You carry out the 'Live Mag' check just before that, with the throttle closed, and the engine idling. The reason you put the mags back to 'on' is to enable the idle system to drain when you put the Mixture to ICO.
That's always been my procedure (for Continental and Lycoming), never a problem.
The sense of abuse of aircraft piston engines seems entirely misplaced by some folk. An idling engine is pretty hard to abuse in the short term. What I would really like to see stopped is throttle jamming - in either direction. Pilots sitting on the runway, and jamming it open, or at altitude, jamming it shut. That's where expensive damage is being done! If you handled the control wheel or stick that way, you'd be damaging the airframe, how is the engine any different?