Gun fit is fairly important, but a progressing beginner will not offer a stable enough platform upon which to judge gun fit. As Ed implies, it will be a while before the shooter has stabilised enough for the fit to mean anything.
So, beginners: Get a gun that is roughly right and go and shoot loads.
More established shots: Get gun fit about right then learn the detail of where it shoots.
As for the idea that perfect gunfit is the answer? Look at Richard Bunning, Chris Childerhouse, Chris Biddlecombe. They wouldn't get through a first lesson without being picked up for unconventional gun hold.