Interesting, I have been looking into this as well now that NC is requiring W-2's/1099's to be efiled. I have only been able to find this on the W-3 instructions:
Who may sign Form W-3. A transmitter or sender
(including a service bureau, reporting agent, paying agent,
or disbursing agent) may sign Form W-3 (or use its PIN to
e-file) for the employer or payer only if the sender satisfies
both of the following.
• It is authorized to sign by an agency agreement
(whether oral, written, or implied) that is valid under state
law; and
• It writes “For (name of payer)” next to the signature
(paper Form W-3 only).
Use of a reporting agent or other third-party
payroll service provider does not relieve an
employer of the responsibility to ensure that
Forms W-2 are furnished to employees and that Forms
W-2 and W-3 are filed with the SSA, correctly and on time.
Seems we can make up our own agreement and have client sign to keep in our records. Curious what others do. I certainly would not want to submit anything without written authorization.