If you want to have a score in a given ability at character creation, you need to buy the virtue. If you plan on raising it later, you don't have to. Being part of a tradition is a bit of a backgroundy thing. Stating that your character is member of such or such tradition can be as simple as your SG agreeing to it. Joining a tradition later will at the very least require a story.
Note that you can only ever be member of a single tradition, and that hermetic magic is not a tradition - so a magus can join a hedge tradition later without penalty while a hedgie will have trouble getting initiated into hermetic magic. I was confused at first, but Erik kindly clarified things for me.