You can go either way and not really be wrong, but I personally think you should focus on getting your entire team to 5 and then after that, in future rebates, jump from 5 to 7, and then from 7 to 9 if you happen to do well enough to get adv refines in sage.

It's been my method and I'm pretty happy with it, I'm not really a fan of frontloading refines onto one ninja, it works for some teams though.