Medicare A comes without charge, so you might as well sign up for it. Medicare B is optional. You are not required to sign up for it, but you should have an alternative available if you don't. The decision depends on your own particular situation. But for every year past 65 that you don't sign up for B, the premium does go up 10% if you eventually jump in.
Thanks for the clarification, i.e. the difference between parts A & B. In my mind I always lump them together, which I shouldn’t.