That's really a loaded question and you'll get as many answers as there are growers. From some tissue samples of cannabis plants, the need for Nitrogen does not go away when a plant reaches bloom. It actually goes up slightly, about 6%. The need for additional P and K goes up 50% and 41% respectively, but the need for P and K are way smaller than the demand for Nitrogen in the first place. I'm a proponent of supplementing your grow component with some bloom, but not abandoning the grow altogether in favor of bloom. I'm sure you'll have others that do differently with their grows. There's a lot of decisions to make in this growing game and you'll have to pick which philosophy you subscribe to.