I myself don't go out of my way to materia my gear unless I've reached the point where I'm picking up tomestone/crafted gear, but applying materia is as easy as selling all the useless mats you get from random mobs at a shop. Just check what you need before hand, press a few buttons, and you've got some stat boosts on your gear. There's no reason not to slot materia unless you're going through gear every other level from dungeon drops.