Might, might, might, possibly....

It's pretty simple. If your bottom bracket hardware contacts the face, it needs to be even (faced). Otherwise the fixed cup (or the lockring) will not have contact evenly, all around the face. This can make the spindle line up crooked on the other side, even. If your BB hardware never contacts the face, but only the threads, facing is not needed. Phil Wood BBs, and the NDS of Shimano cartridge BBs do not depend on a faced surface. Dunno about Hollowtech...