Representative payee - Power of Attorney

Here is an excerpt which may be helpful :

Power of attorney is a legal process where one individual grants a third party the authority to transact certain business for that individual. It does not diminish the rights of the individual and does not usually grant the third party the right to manage the individual's assets. It typically makes no finding about the individual’s capability or competence and is not recognized by the Treasury Department for the purposes of negotiating federal payments, including Social Security or SSI checks.

Therefore, if you are a power of attorney for a beneficiary who is found incapable of managing their own benefits, you must still file an application to serve as rep payee.

http://www.ssa.gov/payee/faqrep.htm