Originally Posted by
bradtx
While this and using a file on each face can insure a uniform face, it won't account for parallelism with the opposite face. For an internal cartridge BB this can all that's required.
Brad
I believe the razor blade suggestion was based on a BB that was faced before painting (common in production bikes) where one may (but shouldn't be) concerned over paint thickness.
Otherwise, very few people are good enough with a file to get a flat face by hand, and as pointed out there's no assurance of parallelism or axiality.