Hey Gil, here's one place I read about the 'need' to rotate plugs! Why is there so much conflicting info out there? Either the polarity changes, or it doesn't! They haven't changed how magnetos work in the last few years have they?
http://www.aviationpros.com/article/10381828/aviation-spark-plugs
"Spark plugs should be rotated periodically to even out the electrode wear," Hansen notes. "Magnetos fire the spark plugs with negative and positive charges. The negative charge will wear the center electrode and a positive charge will wear the ground electrode. Just like rotating your car's tires, rotating your spark plugs will make them last longer by using up both electrodes."
and these are the...'aviation PROS' no less!