The easy answer is no, clients (in most cases) are not required to pre-pay project fees on Malt before work can begin. However, there are a few exceptions.
For example, if you've worked with a client many times before and aren't worried about whether or not you'll get paid at a project's completion, then you are more than welcome to start working once the quote has been accepted and the status has changed to: "Project accepted, waiting for pre-payment."
Keep in mind, however, that if the client hasn't prepaid the project fees by the time you've completed the project, you will not receive payment until the client has paid their invoice. The benefit of pre-payment is that you can and will receive funds for the work you've done soon after a project has been completed (without having to pester your clients to pay).
This is typically the case for larger companies that often have 30- to 60-day payment terms for all invoices received. And unless they have opted out of Malt's cash advance program, in most cases, you'll still get paid soon after a project's completion (once the project is marked complete by the client) and the client can pay their invoice according to their company's payment terms. This ensures freelancers get paid in a timely manner.
Long story short: While pre-paying project fees is not required, we highly recommend it to ensure that you're not waiting to get paid long after you've finished a project.