its more of a personal preference. I like to pick a time and day, turn the lights off and start my 12/12 cycle. Others will say that at the onset of flowering, it is beneficial to leave the plants in total darkness for up to 48 hours to trigger flowering faster. Some people even reduce the light cycle day by day, similar to how nature would handle things. as long as your plants are getting 12 hours of light and 12 hours of dark, how you get there is up to you
hope this helps, good luck with your plants.