In our airline the cabin crew are free to start their service when they wish.
If we are expecting the possibility turbulence at lower levels we instruct them to remain seated until we give them a 'double ding' (ie I flick the no smoking sign off then on again). Chances are we will be happy for the crew to move around the cabin sooner than the pax. (eg still in cloud with light turbulence but avoided everything on the weather radar)