According to the current published rulebook...
http://permanent.access.gpo.gov/lps41631/2005/p53004.pdf
You do NOT need to be a gunsmith to have an FFL, however a gunsmith MUST have an FFL.
Declaring yourself to be a gunsmith at the time of application allows you to operate WITHOUT a storefront and without regular hours. HOWEVER, if you engage in buying and selling (in addition to the gunsmithing) then they will require you to submit a new application to correct your status to DEALER, and you will be required to have a store as
on2wheels noted above.
Open that PDF and start reading around page 121 or 123... (thereabouts).