I‘m a new grower but if you water just before lights go out, then you have less chance of burning leaves if you got water on them. hopefully someone with more experience will answer your question
Right when lights come on is when I try to water. My main reason for that is when I water before lights turn off, humidity skyrockets in my grow environment. Although, that may not be an issue depending on your setup. For example, if your inline fan is on 24/7 exhausting air then you probably wouldn't encounter spiked humidity during lights off like I do. My inline fan only runs when the temp or humidity reaches a certain set point because I'm also running CO2. So lights off my exhaust fan doesn't run as much as lights on. But overall it really doesn't really matter what time of day you water your plants. Hope that makes sense.
Light or dark?