Heal is pretty important to level up if you don't want to be stuck carrying around three goldneedles in some of the later dungeons/towers. I have nightmares about an incident that happened to me shortly after I got richard. Haste and Berserk would probably be good to level up so you could cast them 4 at a time and have them all be effective. MP will level up faster when you get your spells to higher level and cast them. That's one reason I've been enjoying playing maria as a back-row magearcher, sometimes I just get her to take off backrow monsters with her magics so that the rest of the party can meet halfway in the middle of the group xD

The levelling system is so subtle it's impressive. You have to imagine that the map was designed like it was on purpose, to force you out of your path and give you chances to level up when venturing place to place. At least if you don't waste all your money on the airship and ferry between palm and poft. If only it didn't take up so much time xD I think it's a better situation than in ff1 though, because at least you CAN take initiative yourself and speed up the process.