You really should. Using a written agreement avoids disputes by providing a written description of the services you're supposed to perform, when they are to be performed, and how much you will be paid.
A written independent contractor agreement can also help establish that you really are an independent contractor -- not your client's employee. Although an agreement by itself doesn't definitively prove that a worker qualifies as an independent contractor, it will help show the IRS and other agencies that both you and the hiring firm intended to create a hiring firm-independent contractor relationship, not an employer-employee relationship. For additional tips on proving that you are an independent contractor, see Preserving Your Status as an Independent Contractor. For information on what to include in a contract, see Use Written Service Contracts for Your Clients.