I used a monk for the first part of the game, but it wasn't all that worth it. I usually try to switch my jobs every time I get a chance to buy new stuff and heal. Not all of them, but many. I beat the game a few days ago... its pretty sweet. The Dark World is awesome.

As for jobs, My final team was actually nearly the same as my final team when I beat played the game on my GNES.

I never once used shields... though there may have been times when I should have. I have tried out most of the jobs, if only for one fight.
Geomancers are the best mid-game fighters. After you get two bells to equip them with, take one and go to town. Their Terrain skill isn't all that good except in some key places, Most of them I'll need to play the game again to find... But their attack rate is amazingly high. My geomancer in many cases was either Out-damaging, or matching with my Dragoon. After I got access to the Black Belt it became less useful... but Geomancers are far better than they look.
Vikings are probably the same way, but I only used them for the fight against Goldor, so I wouldn't know. Then I changed my entire party to Dragoons for the fight with Garuda, and then switched off two of them for the Dark Knight, and Geomancer.
I switched between Dark Knight and Fighter a few times...
Fighters are amazingly good.
I found however, that the Knight lacked hitting power until later in the game.
The Thief is useful only when you don't intend to fight any battles. Either when you're mini, or when you're trying to fight something much higher level than you. Stealing items its cool, but unfortunately not all that useful in this game. Though I've heard you can steal Zantetsugan from Odin... I could be wrong about that...
Black Mages are cool, but only when elemental damage is crucial (Hyne), or when physical damage is severely lacking (mini). Most of the rest of the time, they'll do what they need to, but don't have the right damage output to be all that useful. They're far more limiting than they are useful. Unless you know what everything is weak to. Then I could see them being good.
Hunters are the same way. Their damage output is pretty decent, but if you give them the right arrows, their damage will skyrocket. And its nice that they attack from the brave position of being out of range.

yeah... I could say more, but I should save it for my walkthrough...