It varies state to state. It is has been statistically found that those that are represented are more likely to win their case. If you have the money, hire someone.
Given the intricate nature of SSD eligibility requirements and various loop holes in said regulations one would be best served by hiring an attorney or other source of representation. If an attorney is out of your financial reach you should consider legal aid centers for assistance in insuring all of your documentation is in order.
You can represent yourself for SSD. Review your social security files and all medical records. Self-representation is not recommended, however. Speak with an attorney before making this decision.
While it is perfectly legal to represent yourself in a SSD hearing, it would be a smart idea to have an attorney on call just in case if you have questions or are stuck on paperwork.