Another point worth making is that earthing the magneto is not "fail-safe" - another reason why props should always be treated as live!
If, when you do a mag check, there is ZERO drop when you select an individual mag then this would point to a live mag, in which case I would advise a check at idle power and momentarily selecting off and making sure the engine ceases to fire. If it continues to fire then you have a live mag the defect needs fixing asap!