It may not be that carbon build up occurs ON the glow plug... it may exist IN FRONT of it.. thus does not come out with the plug... or show that it is needed by examining the plug....
Since this is one of those " Once in a quarter of a million miles" deals... and a special tool is made for it...I say,,, if the hassle of taking the plugs out is already expended... why not clean it out while you are there ?
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