Personally, I'd do it first thing to get maximum hours of light hitting the lower nodes. I don't know if there's any scientific reasoning for one way over the other, but to me more light seems like a good approach.
Edit to add: Realistically, it probably won't make much difference. Tie her down and you'll start seeing new growth relatively fast, adjust tie-downs as needed.
I trained one when it was 15" and another at 4', but do it in stages. There won't be any warning when a crotch snaps. When this happens they can be healed, but growth will be stunted for a while.
i always did and do mine all times of the day id say it dont really make a difference if ur searching for long term results but like super croppin id say its a technique u should rather do in the morning so gives time to kinda heal...do whatever fits ur schedule id say dont have sweat it.... its not something as important as a timer for ur lights...
Fuck ya that is, but thanks everyone I did it this morning an the northern border kush loved it right now it looks fucking sick, but the other one I did it to, it looks like its a lil stunted