No, it's not required. It's helpful, but the actual impact it makes on your damage is minimal. Base weapon damage, main stat (dexterity in this case) and personal skill are all more important than the materia you put into your gear. But as was said, if you have the materia, you should consider using it.

For ninja priority, yes, critical hit should be your main focus, and if a piece of gear is capped on that stat, then add more direct hit.