really depends on which position you play, by in large you should always have things to do no matter how the open play is unfurling.
as i a hooker i still had line outs to throw, and scrums to pack and you always have to be chasing the play, even if the breakdown clears before you get there because, if i did make it was my job to get in and clear out or defend the ball
watching the play to try and pick i line that will actually get you to the breakdown after the next one is the key. if one of those backs can have a couple of forwards right behind him when he goes to ground then you can defend the ball whilst keeping some structure. if they have no one and other backs have to go in to clear out then the flow and game plan start to break down