I found fighting just with Vaan and Penelo extremely tedious. And you are stuck around Rabanstre too. Bo-ring!

I recommend only getting to level 10 before the Palace, then rushing to Bhujerba. You get Larsa there but you won't need him at too high of a level, besides you will get him again later.

You don't want to get Vossler, your second guest at a too high level because (SPOILER)you will have to fight him

After Vossler it is a good idea to level up. You will get Larsa again and later Reddas. And you have free roaming through most of Ivalice The best time to level up is then or after you get to Balfonheim IMHO.

But I finished the game at level 64 and I'm a klutz! You don't need to get to level 99, only do it if you want to.