It seems to me that it's the opposite and that close ups are your safety's Sometimes in a master there may be a continuity flaw, or something doesn't match. In close ups of one actor at a time, there is less likely for that to be noticeable, so why is the master the safety? In my experience, it's the most risky. Not that I object to it all, I love mastershots, depending on what part of the scene, and what mood I am going for. I just don't quite get the safety thing.