There was an earlier discussion on the list about this:

62.1-2007, Section 5.4 states "Mechanical ventilation systems shall
include controls, manual or automatic, that enable the fan system to
operate whenever the spaces served are occupied. The system shall be
designed to maintain the minimum outdoor airflow as required by Section
6 under any load condition."

62.1-2007, Section 6.2.6.1 states "Ventilation systems shall be designed
to be capable of providing the required ventilation rates in the
breathing zone whenever the zones served by the system are occupied,
including all full- and part-load conditions."

These are generally applied to mean that the system is not required to
operate when spaces are unoccupied. Yes, the standard does not
explicitly state that unoccupied ventilation should be zero but the
standard does not explicitly state that unoccupied ventilation can't be
zero. Section 5.4 and 6.2.6.1 only require controls and ventilation
rates "whenever the spaces/zones served by the system are occupied" so
it can be implied that the requirement doesn't apply when the
spaces/zones are unoccupied. There's a fine interpretive line there,
but many of the official interpretations on 62.1 also lean this
direction. If in doubt, put in an official interpretation request
specifically on this with ASHRAE on the topic to clarify.

Jeremy R. Poling, PE, LEED AP+BDC