A utility knife is less likely to remove metal than a file. A decent file is an actual metalworking tool and can remove metal and alter the faces. That being said many frames did not come faced from the factory. A minor cost cutting measure few notice until it’s time to rebuild the BB.

However, there’s another solution to facing bottom bracket shells, using a cartridge square taper BB. The bearings come aligned in the cartridge, the unflanged cup doesn’t even touch the faces, and the flanged cup just bottoms out and is held in alignment by the other cup. Facing BBs for cartridge BBs can be detrimental even because raw steel is exposed to the elements. They also require less frequent service, preserving your crank tapers, and replacement is easier than rebuilding.