Unless otherwise specifically stated, the authority given to sign off any skills test, be it Tote-N-Chip, or rank advancement, even a MB*, ultimately rests with the SM, and said authority may only be delegated by him or her. In fact, it MUST be (is supposed to be) specifically delegated by the SM. If, and only if, the SM says that so-and-so is authorized to sign "it" off, whatever "it" is... then that's "it". It is a widely (and wisely) accepted practice to allow senior, experienced, capable Scouts sign off on Tote-N-Chips or junior rank requirements, for example... that's fine. But, it falls within the perview of the SM's authority (and responsibility), not tradition.
(*) Yes, there are District/Council Advancement Committee required, approved, authorized, yada, yada, yada, MBC lists... But, the SM still retains the authority to select the MBC, should he so choose. However, once a Scout starts a MB, the SM's authority has been, in effect, delegated to the MBC, whether the SM selected the MBC or not becomes moot, and he (she) is obligated to accept the results. See:
http://www.meritbadge.net/phpbb/viewtopic.php?t=490