Facing "needs" depends on the design of the bearing seating with the frame. As example if the BB shell uses press fit bearings that are held by the shell's ID and located by an internal shoulder/seat the facing the shell's face will do nothing to effect the bearing, for better or worse. But if the shell is a threaded design where the bearing "cup" is tightened against the shell's face then, yes, insuring the bearing seats square to the shell's axis is important. So to with headsets. Bearings that seat within the HT's ID (like an integral style often does) has no association with the HT's faces. But cups that press against the face/ends of the HT do benefit from facing.

Giant TCRs have had a few differing designs over the years. Which do you have, WRT the bearing designs? Andy (as always wondering about the details not mentioned by the OP)