There's no harm in you working on levelling your Arcanist now or waiting until later on to do it, it all depends if you want to prioritize the main story over it or not.

Main thing to keep in mind that when you hit level 30 Conjurer you definitely want (or rather need to for the sake of people you'll be dungeoning with) to level Arcanist to 15 so you can access the White Mage job.