Hey lupine... hate to burst your bubble because it really isn't as rough as it sounds, but many of the shops (I know) that actually have a tool prefer to use a file... especially the older more experienced guys. I remember when I was doing my apprenticeship that we were actually TAUGHT to use a file over a LONG period of time and it was and still is an accepted part of engineering/toolmaking with very very accurate and flat results possible if you take your time and hold it flat against a well-held surface (clamp that bike down well). We machine the surfaces of disc mounts BEFORE fitting them and hopefully we shouldn't have to touch them after that, but most of the mounts I've seen (both factory and custom) have ALWAYS needed shimming if you want them perfect and it doesn't seem to matter whether the magic tool, a file or a laser guided stealth grinder has been run over the top of them or not.
Cheers