While a White Mage would be super useful for the next part, it's not for the party preservation. Though a White Mage is pretty much the MVP for Magissa/Forza battle.

Honestly, I don't actually use White Mages terribly often if the class choice is left to me, and while it might be awhile before I can acquire White Wind or the Chemist class, I don't think I would absolutely need a WM until Walse for the Shiva battle since she can be a real pain if you fight her early enough. I'll definitely need one later but I feel I can make it through the early sections without much trouble as I don't feel FFV gets particularly difficult until Galuf's World.