I dont think it really matters too much.
what I do is,,,
On the last day of Veg, When the timer shuts the lights off, I re-set the timer so it doesnt turn the lights back on until the time i re-set it to. Sometimes leaving the plants in complete darkness for about 14 hours or more, Depending on when I want to start the 12-12 light schedule.
Example: In Veg I had the lights on from 10am - 4am,,,
When i switched to flowering, my lights are on 6pm-6am,,,
So the first day i switched over to flowering, the plants were in darkness for 14 hours.
Some people even leave their plants in complete darkness for 24 hours on the first day of switching to flowering.
Its said that doing that jump starts your plants into the flowering cycle.