A load of Bollocks, a rough running mag ( as opposed to a U/S mag, lead or plug ) is generally one of two things :
1) a little moisture in the Mag causing arching, or,
2) Carbon / Oil deposit on the plug, this is best removed by increasing the burn temperature in the cyclinder, for example run up to full power and gently lean this mixture for about 30 seconds.
So in other words, having run the engine up to do the prop check has infact increased the cylinder temperature and probably burn't off the crap on the plug.