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I did strip out one 5mm threaded eyelet and had to tap it out to 6mm. A clean hole won't be any different from a threaded hole except that you can't strip out the threads. The resistance to wobbling doesn't come from the fit of the bolt in the hole. Were that the case, the hole and threads would get mangled up. No, the resistance to movement comes from the friction between the rack or fender mount and the bike part which has the eyelets. That's the reason that bolts have to be tight and stay tight: to create enough friction to prevent movement. The bolt just needs to be large enough to have enough thread area so that it can be tightened to an adequate torque to prevent movement. It doesn't need to fit tightly in the hole, though that would be better because then the bolt would be larger.