It doesn't really matter. Switching to 12/12 is fine.

If you are worried about herms, Dutch Master makes a product called "Reverse" that you spray on your plants in week 1 and week 3 of flower and it kills any male flowers present. I used it on my second grow after having a rogue herm somewhere on my first grow and I didn't have any this time. Its only $25 so it might be worth checking out.
dejayou30 Reviewed by dejayou30 on . can I go to 18-6 or do I just go straight to 12-12 I don't want my plants to go hermies agen. so I was worndering of 18-6 would work ok for the first few days. then to 12-12. or go straight to 12-12 when I am ready to flower. Rating: 5