Bumblebetuna's an idiot, so you don't really have any good support. Try listening to the level 75 white mages who actually have a clue what they're doing before bragging about support. Oh, and before anyone wants to bring it up, don't listen to Shaunessy either, he's an idiot too.
White mage can tank at lower levels, because there's really not much difference between the jobs at lower levels. The mobs are fairly weak and hate will bounce around enough that there is never one tank. There's also practically no way to hold hate for any job at those levels so it won't matter that you're a sucky tank. You'd have to sub warrior to be any kind of tank, but if you're doing that you're already playing the job wrong.
If you're trying past 30 to tank, you're an idiot. White mage is a support job, if you want to be a tank play a tank job. Yes, they can equip weapons, but they can only equip clubs and staves, neither of which does damage worth a damn in exp, nor is their skill level high enough to really warrant using them in exp. White mage is a HEALING JOB. We have the best heals in the game, we're the only job with regen 2/3, cure 5(which is ridiculously low hate), raise 2/3, -na spells, erase, without the depenency of a sub job. I have kicked whm's who refused to be healers before, and I'll definitely boot any whm who tries to tank and not heal.