I know many people leave their plants in total darkness for 24-72 hours right before switching to 12/12 which helps with the plants hormones and jump starts the flowering phase. Once you start the 12/12 cycle, you really don't want to mess with the plants once the lights go off. If you absolutely must do something during lights out I'd say do so quickly and make sure no light is directly hitting the plants. But usually during flower you don't do anything until lights on. You risk getting a hermie. Total darkness, no light leaks of any kind.