Having personally done more 103 head jobs than I could ever count I have to say that i have never had a problem with a head that has been machined within the allowable thickness. Never had cam bearing damage, never had to shim cam towers, never had a cam that wouldn't turn freely after the head was torqued. Pulling the guides, planing both sides etc is way overkill for this engine. Sure it would be nice on an ultra high dollar custom race setup but a "fatal mistake" not to....No way.