There are 3 spots:
Adventureland, HOP and Tomorrowland.
Where ever they make you stay from 6:00p to 7:00p their photo stop will be open at 6:00p (this year it was Adventureland; in the past it was Tomorrowland) and the other 2 photo areas will not start till 7:00p.
I prefer getting it over with in an area that's just opened up. Some wait till late when it's died down a bit but I'd be afraid my kids would look messy or be asleep by then.