I don't know if there is a hard and fast rule on when and when not the front end crew turns on and off the seatbelt sign. I've been on flights where it's never turned off and I've been on flights where we have to call up front to tell them to turn it on because we are getting tossed around. I don't know of any f/a who will tell someone that they can't go to the lav unless they know of a reason that it would be unsafe.