Just my opinion (and the opinion of everybody in the business) but there are no secret squirrels in gun shops. You want that in action as fast as possible. And it may not dissuade some extremely rare crims hellbent on death, but it will make some inbred methhead think twice.
BTW, even the extremely rare crims don't run into the first gunshop they see and walk out with a .22 and a lever action BB gun. The dumbest among them send somebody in in advance to see what they got. And when they got 3, 4, 7 guys carrying guns they tend to move on.
If gun shop employees only carried concealed there would be a heck of a lot more attempted gun shop robberies. Nowadays they are rare, and people die when they happen, every once in a while one of the good guys . They would be much more common with concealed carry.