No, a power of attorney need not be filed in order to reply to a patent Office Action. 37 CFR 1.34 states that a patent practitioner acting in a representative capacity may sign a paper in practice before the USPTO in a patent case. The signature will constitute a representation to the USPTO that the patent attorney or agent is authorized to represent the patent applicant. In filing such papers, patent practitioners must set forth their registration number, name and signature.
A power of attorney will ultimately be required to participate in phone interviews for elections in Restriction Requirements or approval of examiner’s amendments. (MPEP 408)
Interviews with patent practitioners not of record in an application that has not been published under 35 U.S.C. 122(b) with an attorney or agent not of record who obtains authorization through use of the interview request form will be conducted based on the information and files supplied by the attorney or agent in view of the confidentiality requirements of 35 U.S.C. 122(a). (MPEP 405)