Don't get me wrong, doing leves is by no means a wrong way to do things, that's what they are there for. However, if you want to rank quickly and efficiently, saving your leves for when you need more SP rather than less SP, is the best bet. If you use all 8 of your leves today, and you reach 16+ you'll be at a crossroads and -have- to grind mobs and do Behest for the next 30-odd hours. If you grind/behest your way to 20-ish, you have all 8 leves waiting for you at that rank to boost you further, plus the higher your rank, the more likely you are to run into other players leve-linking, etc.