Yes it's okay to wait behind the line and that's where you should be stopped when waiting at a traffic light. I see some people stop clear out into the walk way when they should be stopped behind the walkway at the light.
You shouldn't wait too long at a green light though as people will get impatient and you'd be holding up traffic.
You should make sure that it's safe to do so then you should make your turn as soon as possible after the light turns green.