Final Fantasy Question (Should I level up my characters)?

You should be fine. Larsa is a big help. If you can get every one an Embroidered Tippet. This doubbles your EXP. You can get them in Old Archades from the Seeq merchant. I went to Ogir-Yensa Sandsaa and ran around the platforms to build up big chains of over 200. You get Montblanc in your Sky Pirate's Den for getting a 50 chain in battle. Later when you go back to the mine for the Esper Zodiark the beast are in the high 40-60's. I'm at the end of the game right now. Just doing all the side quest. Vann, Balthier, and Fran 98 and Basch, Ashe, and Penelo 86.