I was reluctant, but after speaking with some frame builders, was convinced it's not a problem. However, I would not drill a main tube myself, because it should have a welded or brazed reinforcement-- which I'm not capable of doing myself. The hole on the downtube near the headtube lug has a brazed-on reinforcement: a 5mm bottle cage boss with the threads drilled out. The only other hole drilled is at the bottom of the bottom bracket shell; this has no significant impact on the structural integrity of the frame. Some BB shells have huge sections cut out for weight savings. Everywhere else on the bike, pre-existing opportunities for internal wiring were exploited. The existing fork and fork crown vent holes were utilized for running the wire inside the fork (which was no small feat), and the chainstay bridge hole took a 6mm threaded adjuster barrel without modification.