I was taught to inlet the bbl minus the breach plug, then install the breach plug and inlet it. It's just easier to get things squared up. There are builders that inlet it all together.
For a pre-carved stock you don't have much flexibility to move the placement of lock due to the mortise already cut. Some builders inlet the bbl minus breach plug, then inlet the lock to ensure the touch hole will line up where it should. If it doesn't you might be able to adjust the barrel back if needed then install the breach plug and finish inletting it. If you inlet bbl and breach plug (which ever way) before the lock you might have a real problem if your lock/touch hole doesn't align.