I can't quite tell if this was mentioned, but, though you can change your jobs to any class throughout the game, it's best to keep your classes in certain catagories.

The reason for this is that, in the beginning, you get specific classes:

Fighter: strength
Monk: HP
WM/BM - MP and White or Black Magic power
Red mage: cumulative three

If, later in the game, you decide that you want your characters who were fighters to be mages and mages Fighters, the initial stat boos of changing jobs is all you'll get, and the stats will rise accordingly from there. So... If you wait until the end of the game and turn all your mages into Vikings, for example, they would be supremely underpowered, but, on the bright side, they'd be able to cast excellent spells. Except they can't. And then after levelling up a bit and turning them back into mages, their power would have increased, which is useless. You might as well call a lv. 30 character a lv. 26 character, because 4 level ups were useless.

Got it?

In the beginning, breaking the rules a little, I usually make all my characters monks for the first 5 levels if possible, just to get a HP head start.