A good rule of thumb, is until rank 20, your optimal wont matter since you can get past those levels so easily

So id say go straight to a rank 20 or so item if you are starting a job

HOWEVER, afte that, you want to stay as close to optimal as possible(as far as not equipping stuff over your level)
having stuff under your level isnt gonna hurt you too much as long as you arent too high over ti(10+ levels)