I have been thinking about doing this for a while now, but think that it would pull out my supplimental co2, so I dont know.
When the lights are off the c02 should be off, as plants for the most part have no need for c02 during the lights off cycle.
...so it wouldn't matter if your venting fan or any fan is running after the lights go out, it's a waste of electricity imo.
Myself, All my intake and exit airflow stops when the lights go out. As long as the roots don't get to cold when the lights go off, it can't hurt to leave the fans on (this is applicable if the fan running is an intake fan connected to the A/C or what not, bringing in colder air than the ambient air when the lights are out.)
The only time I deviate and let cool air into the growroom when the lights are off, is when I grow blueberry and want to see more blueish tones to the foliage. (The same way mother nature does it outdoors, I just make sure I have a small fishtank heater in the resevoir to keep the water at a constant temp) If no blueberry, the fans shut off when the lights go off and no fishtank heater is used.