I think this came about because so many of the early bottom brackets died in double quick time. People were looking for an explanation (someone to blame) and they picked on shell facing.
Personally, I think the nylon washer is compressible enough to take up a bit of misalignment. Any more than that and it’s a thread issue that facing isn’t really going to help.
A lot of early demise on HTII type bottom brackets is due to people over tightening the preload cap. Oddly enough, the same thing happened with headsets when things first moved over to A-headsets.
The deep groove ball bearings in bottom brackets don’t like lots (or any) side loading.
In short, fit the bottom bracket properly and don’t worry about facing.