Synching your CO2
with your lights is the safe way to go. Plants do not uptake CO2
during lights-out so it is not necessary to supplement it at that time.
On the other hand, you will experience larger pH swings with the CO2
off at night. When the lights come on in the morning the CO2
will have to catch up with the amount that out-gassed during the night. This shouldn't be too much of a concern though.
If you have a pH controller you can leave your CO2
on 24-hours-a-day without any concern because it won't let the CO2
reach dangerous levels.