There are a couple of different types. Some rotate the full 360° without a stop, which is generally annoying, and others rotate and lock at each 45° or 90° points. Or at least those are the two types Ive run into.
I prefer the ones that lock at the 45° points, as you get more adjustment options and the sling doesnt get all bound up.
trackskippy is right. It's one of those "it depends" situations. I was watching some videos on YouTube about the installation of the QD cup attachments for the Magpul Hunter X22 stock. In the video, he shows that the front swivel only rotated 45 degrees while the rear swiveled 360.