Facing may well not be necessary, depending on who made the frame, as some are properly checked before shipping.
Facing is preparing the external ‘face’ of the BB shell – definitely required for external bearing BBs. Reaming is where the threads are prepared, in order that the BB screws in smoothly and does not get damaged. All of this can be affected by paint on either the face or the threads, but a plain metal frame can still need faced and chased (or reamed) as the tubes might not have been finished all that well.
Ti is significantly harder than steel or aluminium and so will take miles more grinding to get right – so the bike shops are right. Facing tools are stupidly expensive (£600+), so blunting them on your frame might cheese them off a bit!