First determine if your frame needs facing. If the frame is new and the BB shell faces are shiny smooth with no trace of paint on the Drive Side (DS), then it's likely been faced by the manufacturer. If it has paint on it, or you're hanging new parts on an old frame, it should be faced. Some conditions apply. If you're using a BB that has an adjustable chainline (no shouldered cup on the DS) then you don't need to face the shell. IF you're using a BB that uses a shouldered cup on the DS, then you should face the DS only and leave the paint intact on the non-drive side (NDS). If you're using a BB with external cups (new shitmano or X type) then you should face both sides of the shell. In all cases you should chase the threads in the BB shell if you have any tight spots or resistance to inserting your BB cups.
Any "pro" mechanic who tells you that you don't need to C&F your shell because the tool is too expensive is definitely not "pro." Run away.