Had a live mag a couple of weeks ago, found it on a drop check and confirmed by switching both off and the engine ran on. Can't see any real reason for switching off both mags, but having said that we used to do that at school I used to teach at, always at idle though and for a fraction of a second, just enough to hear the engine miss a beat.
The problem with the dead cut check - in a training environment anyway - is that students tend to be a little ham-fisted and it was not unusual for them to get the key stuck in the 'off' position and wrestle to try and get it back to 'both'. You had to jump in quickly then and tell them to leave it or you got the aforementioned POP as the unburnt fuel went off (this was much worse if the engine wasn't at idle). Better not to in my book, unless to confirm a suspected live mag.