It does not matter but but dew does tend to form before before the sun comes up. You may want to read up on the “naturalistic” approach to hydration via high humidity at night. I hope I’m not butchering that, @Decadancin could explain better than I...
I always did lights, then mist about 30 minutes later but now I actually set the mister for a short (20 second) burst about 10 minutes before his lights come on. In the wild they would have morning dew to drink from immediately upon waking and the idea is to have that water that would be naturally available to him, available to him in his enclosure as well.
Im also a mister before lights on(its my alarm clock), but i really dont think it matters since odds are you will be misting 4-6 times a day anyway, even if its just to keep the humidity up.