You're free to use Prep however you wish, of course. If you want to remain in the running for the major Prep awards, you just need to go no more than 72 hours between logging assignments (see pinned post) - sounds like you already know this. But in terms of the Prep awards, once you're done, you're done, even if it's a couple days or a couple weeks early (in other words, the 72-hour rule is no longer relevant after all assignments have been posted). It's also okay to do them out of order if you want to.