I use a controller, along with a drop checker for verification. So far my tank parameters are constant. I think a lot of people think a controller is an easy way out but in reality it is more difficult and harder to properly set IMO. I dont need to run a controller, its just an added feature that I happen to have and performs other functions as well. I could easily set it to monitor and have it alarm if the pH gets beyond a limit or shut it off at low limit if something were to happen like my needle valve getting bumped. It comes in handy also since my needle valves seem to wander a bit. With the controller I can set them higher than I need, and the controller just shuts em off for a while if it gets to high. if I ever get better NVs then I'll probably use the controller more as a monitor and emergency shut-off.
But to answer the question. I get about a 1 point swing from day to night. My controller is programmed to maintain pH from 45min prior to lights on until 15 minutes before lights out, after that the solenoid is set to shut off. I have enough surface movement that my tank appears to totally off gas by morning. So my drop checker is dark blue in the morning, then is green during lights on. With the rate of CO2 I am using, its definitely a good idea to shut it off at night, that will give me about 3X the service interval, plus gives the fish a break at night. I could target a lesser co2 level overnight, like say 15 or 20ppm, but after talking with Tom Barr he seemed to think it was unnecessary and just shutting it off is usually the way he goes.