I have a character working up to be a Summoner, Black Mage, White Mage, and Scholar, which means leveling up the disciples of magic. But the Black Mage requires a lvl 15 archer class. Is it within my best interest to keep leveling my archery as well as the magic or just ignore it and focus on the magic classes?
I took archer to 34 for Quelling Strikes, useful as a BLM
As Meklore said, Quelling Strikes is useful. Basically, you'll want to look at any secondary class and see if there are any abilities they get that can be used in your job that seem useful. Quelling Strikes at 34 is the highest-level skill you'll be able to use.
That said, I am wondering if they might put some more above 50 in the expansion. Still, it's not really worth worrying about that until then if you're not enjoying archer. And even then, most of the time these skills are "nice-to-haves."
Once you've levelled a second class to 15 for job unlocks, there's no requirement to level them any further unless you want a specific cross-class skill. Check out one of the guides, see what cross-class skills are recommended, and if any of them are particularly essential. Once you switch into your job at 30 you're pretty limited in the number of cross-class skills you can equip anyway, but generally most of them are only really essential once you're getting into endgame stuff, you can do fine until then without them.
Level Archer to 34, ignore making it bard if you dont want to deal with PUG to 15. There are points like when a BLM flare spams, where not having that ability up could lead to a quickly dead blm. If you plan to raid with BLM you'll be told you have to have it.
Eventually level arc for quelling strikes but don't stress about it until you are 50 and decently geared on your blm.
I suggest leveling classes if you want... all the way to fifty? Only if you want
Like for instance I only leveled lancer for Invigorate and Blood for Blood. I have no intention on leveling it any further, since I don't like it.
Also MRD, I'm aware a few cross-class skills would be nice for my GLD but there's no way I'm going to get a max marauder since I don't care about playing it.