It depend how you level your job.

Bonus level are up until the end of you next lvl, but it's used only with the xp from the mob.

So if you make leves or fate, it doesn't consume you bonus.

Personnaly, i level up my job with leves, and because you don't kill so much mob with them, i do not use a lot my bonus this wa (and it help because i use it so with my crafter job)