You don't need them until you are ready to level it to 11. You can unlock levels early, but I never do. I can control when it gets leveled up by leaving it locked. Now, with Zeus, it doesn't really matter, so you could unlock it if you want. It doesn't change anything. It will just allow someone else to put the last FP on it when going from 10 to 11.
One thing I will suggest- I do not trade BPs when leveling up. (I guess I would consider it for an Arc if they were really lopsided- like 7 of a few and only one missing.) You need a full set to level up each time after 10. You are essentially throwing one away by trading. For the next level-up, you made need one of those you just traded in. I'm sure many players trade them in. I don't.