If the gunsmith doesn't know what the proper threads are for a AR-15 flash hider, then I wouldn't trust them to do the job.

With that said, it depends on what flash hider it is, and how you want it attached. If you want/need it indexed without crush washers or peel washers, then they will need it. If you are getting it permanently attached, then they will need it. Otherwise if you have the tools to properly clamp the barrel in a vice and install the flash hider, then they do not need it.