I'll have to disagree with part of this post:  With a standard 6-cylinder magneto system all plugs receive alternating polarity, so rotating the plugs top to bottom or cylinder to cylinder won't do anything (I'm 92.35% sure :-).  On a 4-cylinder system the plugs will indeed receive opposite polarities, so rotating the plugs will work if rotated to one step in firing order.  On a 6 the mag rotates at 1.5 times engine speed, so each plug will be fired in the opposite polarity every time, so rotating the plugs will have no effect.  With most electronic systems, such as the Lightspeed, 2 plugs are fired from each coil, reversing the polarity on half of the plugs, so in this case you could presumably rotate the plugs and gain something.  But in this case the plugs would be rotated to a cylinder opposite the firing order, not adjacent.  I think the idea of rotating the plugs was to equalize any long-term effects due to temperature and oil consumption variations between cylinders.  In that way all plugs will presumably be ready for replacement at the same time.  Is that worth the trouble?  I don't know, but I don't bother.
Gary Casey
ES 157, Lyc. 6-cylinder, one mag and one Lightspeed

another issue to consider is the direction of electron flow. I was taught t= rotate the plugs on servicing. I was told to advance each plug on reinser=ion one hole. as example from cylinder 1 top to cylinder 3 bottom. then 3 =op to 5 bottom etc.. etc.  What does this accomplish? It reverses electr=n flow from center to outer to outer to central thus reducing the wear one=the center. ie it ovals out slower prolonging the life of the plug. Only o=her tip is to be sure and use antisieze (One small drop on threads and a t=rq wrench to accurately tighten to 35 foot lbs. (important)