First playthrough I didn't pay attention to the role bonuses at all because I didn't know about them enough to manipulate them in my favor. Beat everything in-game and got platinum as well as beating all DLC Coliseum fights. Pretty much the moral is there is no such thing as "completely screwing up" the crystarium, but there is a thing as optimization to get the most out of it (which will make everything a LOT easier to accomplish).
As others have pointed out you can minimize future damage, but the damage that has already been done is there forever. Yes, Commando and Ravager are important roles to level, but not SO important you "need" to skip out on stat bonuses from those roles just to grab the abilities. On small nodes, yes, leveling roles OTHER than those two is very beneficial. Early levels in Saboteur, Synergist, and Medic roles can have a huge benefit for early boss fights that can prove to be challenging for people playing through their first time.