I'm not sure that it really matters, from what I was reading...Since you get bonus XP for fighting/FATEs for all secondary jobs depending on how high your main level is, so one could argue that by doing all the quests possible on your main (assuming the XP is worth it), you'll level up faster and therefore when you go to level up your secondary job(s), it'll have more bonus XP.

On the other hand, you could argue that a level 15 doing a level 3 quest would give little to no XP and be more valuable for starting....

So the question is how much XP do those low level quests give for a high level job?