Even Hughes said they aren't 100% certain and that everybody is different and so all they can do is closely follow the player and then adjust if it doesn't seem to be working. But it's pretty normal in all walks of life that you would start with a small and once you prove yourself capable gain additional responsibility. In Slaf's case he spent roughly 18 games on the 4th line, followed by another 18 on the 3rd line, and now 21 games in the top-6. Was it imperative that he played those first 36 games in the bottom-6, maybe not, but I can't see how someone would say that's a bad way to develop.