Yes, you do -- and here's why:
A will is an essential back-up device for property that you don't transfer to yourself as trustee. For example, if you acquire property shortly before you die, you may not think to transfer ownership of it to your trust -- which means that it won't pass under the terms of the trust document. But in your will, you can include a clause that names someone to get all of the property that you haven't left to a specific beneficiary.
If you don't have a will, any property that isn't transferred by your living trust or other probate-avoidance device (such as joint tenancy) will go to your closest relatives in an order determined by state law. These laws may not distribute property in the way you would have chosen.