When I have removed glow plugs, I have never seen significant carbon buildup on the tips, and the plugs were very easy to pull out....once unscrewed, they wobble in place and almost fall out. In such cases, I dont believe it is necessary to ream.

But if you see heavy carbon buildup, or worse, if after unscrewing, the plug is still held in tightly by heavy carbon around the tip, then reaming is necessary.

There is a special tool which screws into the glowplug socket and reams the glow plug hole while it is spinning. I dont think this is really needed....I don't see why you cant use a hand-held drill bit about the size of the glow plug pin, or slightly larger to do this (it is only carbon that you want to remove). While I havent seen any formal procedure for reaming, I would think that the way to do it is remove both the glow plugs and injectors, and vacuum out the pre-chambers via the injector holes allowing air to flow in through the glow plug holes...this will insure that no solid chunks remain in the prechambers.

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)