As far as head, gloves, pants and shoes go you can max stats without spending much, if you're gonna do it anyways why not do it now? While materia V is going lower with the rank 6 and 7 moogle quest rewards, once the supply from Moogles is gone they are going back up, so it makes no difference whether you meld now or later.

Now, chest and accessories? If you already plan to go all out on them you can just do it now and save tools for whatever extra stat you need for 4 stars, or you can meld after we know the stats required for 4 stars.

Either way the gil you're saving by waiting is marginal, and like I said materia V will reach a lowest point then it will go back up, since moogles provide a very limited supply per player.