Do I need to be SDL Trados Certified to accept project work?

Absolutely not.

However, holding the credential proves your level of expertise in the translation technology and will make you more interesting to a potential employer.

Remember that job posters may, at their own discretion and for their own reasons, limit their job postings by certain criteria, including membership level, country, CAT tools, etc. Thus, you may wish to report information about your SDL Trados Certification on your ProZ.com profile, so that you could meet a client's requirements if they have them.

Otherwise, you will see a warning message at the top of some job postings referring to this criteria set by the outsourcer.