Speaking as an engineer, it's standard to perform a 'dead cut' check at low rpm (idle) to ensure that the grounding circuit is functioning correctly. I've never had any backfires whilst doing this (it only takes a second to switch to Both Off and back on again and it's obvious to the ears when the ignition cuts).
I once heard someone inadvertantly 'dead cut' a Dakota at max rpm and switch the mags back on again. The resultant explosion of unburnt fuel in the exhausts could easily have been heard a mile or two away!