Sears owner's manual (model 917.272082): "NOTE: WE do not recommend sharpening blade - but if you do, be sure the blade is balanced." They go on to talk about using either a cone balancer or a 5/8" dia steel bolt or pin.
As I stated before, we got a poor cut after my son sharpened and balanced the blade. Since it is the only blade set we have had trouble with, I think the problem is with the blade and not with my son /forums/images/graemlins/smile.gif I also believe that balance should not be an issue, because of how little metal gets ground off. The manual lists only one blade for this mower. Next time, I will look into the gator blades, assuming I can get a match.
A couple of my friends all have 20HP+ Craftsman mowers and they are happy with their cutting performance. Hopefully Craftsman has changed their blade configuration -- otherwise it is certainly a point to ask about if you're shopping for a new garden tractor.