Would it be smart to sign up for multiple load boards or central dispatch will do it for now?

I would say yes. Even though most of the big brokers still post on Central you can get access to their load boards once your approved and see the loads before they are posted on Central. Also it gets easier to see what is still available since Central updates only twice a day whereas the broker's boards update as soon as a load is assigned. Saves you time from calling to see if it is still available.
Another good thing is with Ready Auto (Manheim Auctions) once you start pulling directly for them your company gets a rating and they will send you direct loads before posting them to the load board and if you deliver quicker than others you move up in the ranking and get loads before the other ranked carriers do.