It's a good idea to start your CO2 roughly an hour or so before your lights come on. The reason for this is simple.
When your lights come on, you want your plants to hit for floor running. What I mean by that is you want your plants to begin growing instantly. As the CO2 build up over the 60 minutes before the lights are on your levels begin to rise (remembering the carbon dioxide has been off for maybe 16 hours) and your plants take complete advantage of this. Good levels of CO2 and lighting (don't forget your fertilisers) = happy planted aquarium.
By contrast, having your CO2 come on when your lights start up, you delay the growing process as it takes roughly 1 hour for CO2 levels to hit that perfect 30ppm level.
In addition, turn your CO2 off 1 hour before your lights go off. There will be sufficient CO2 in your tank for that time for your plants to continue growing.
When do you turn your CO2 on and off?