Do you mean 12/12 or veg under hid? I know ur title says 12/12 but a little misled.
Str8 to flower is a personal preference and personally i dont like it. Smaller plants that really didnt look like they wanted to produce compared to my 3 week veg plants that just seemed happy. Ofcourse the 3 week is bigger, it should be, but the mood of them 12 was great and not so sunny with str8 to 12/12.
If under hid for veg, raise lights, mist for a few days to get them to cope with the drastic change in humidity. New growth starts takin off a week later when them roots get all comfy in their new home.
HoPe this helps.
A2SEED