As other have said, you'll want to level up CNJ (at least to 15, if not 34) right away to unlock PLD. It's very rare to see players above 30 that do not have a job soulstone equipped. So with other classes as well, you'll want to do the job quest when you hit 30.

In general, leveling classes up to 15 is not a big deal, provides a few cross class abilities that other classes can use, and fulfills secondary class requirements for jobs. It's never a bad thing to try out a class or two to 15 if the mood strikes you.