Materia can be safely ignored for the most part. So long as you wear gear meant for you, it will have the main stats you need at cap (or near cap for NQ). It's a waste of money to use it on gear you will replace, and all the high level dungeon drop gear can't have materia attached.

That said, it cam be used at endgame to be the best you can be. You can get crafted gear that's 2nd best (ilvl 90 currently) and attach materia. While the main stats will not be as high, it's useful to ensure you hit accuracy caps or pump a desired trait like crit rate as high as possible.