People should be able to specify what their donated tissues should be used for. However, this needs to be within reason. For example, a heart or organ donor should not be allowed to be racist or homophobic from beyond their grave. Donors should be able to specify what research their tissue is involved in.
I believe that people should not be able to dictate what their donated tissue samples are used for because having to track each donated sample to its owner and having to track the allowed uses would unnecessarily complicate doctors ability to use the donated samples. It may also preclude best uses due to lack of donor's consent.