It's in no way necessary, but it's a good practice to at least pre buff the tank. I typically leave dps and myself naked, especially on sch since the fairy will spot heal for me, but the mitigation on the tank ensures that, at the very least, I won't have to cure bomb at the start of a pull and risk pulling hate. It's very easy to pre buff the tank, just cast it at a mobs last 20% hp or so. If you have decent dps, they'll probably clip your last spell anyways, and then there's no waiting necessary.

On the flip side, it's unnecessary to hold the party hostage until getting buffs for most content. For speed runs, the tank should definitely have a shield to absorb damage while pulling the train, and you should be buffing for raids regardless unless maybe you and the entire party really know what's going on. I personally buff every pull, before every boss, but without impeding dungeon progress.