My professional association, National Association of Social Workers, prohibits the solicitation of testimonials from clients.
NASW Code of Ethics, 1996, Revised 2008, 4.07b:
Social workers should not engage in solicitation of testimonial endorsements (including solicitation of consent to use a client’s prior statement as a testimonial endorsement) from current clients or from other people who, because of their particular circumstances, are vulnerable to undue influence.