As Dave Moulton has pointed out, lack of sightable exposure does not prevent rust (hence bolts rusting inside nut threads). It's a spurious argument.

A new modern bike frame does not need facing with the array of sealed BB cartridges which are available and which don't need accurately perpendicular shell faces to do their job. The external bearing jobs? Well they might be a different story.

BB facing arises from the need to enable smooth running of bearings and spindle in traditional-style BBs. There is only one identifiable symptom indicating a shell needs facing for this style of ball-and-race BB -- a tight/loose pattern when the spindle is turned and the bearings have been properly adjusted.