There is a saying..."If it ain't broke...Don't fix it." And it seems to relate to another one of Murphy's laws...."If you fix something often enough, eventually it will break."
I subscribe to both theories/Laws. And I know hat once you take a good operating bearing out of it's normal working environment, and introduce it to an outside environment, no matter how careful one is, there is always a possibility of making a problem occur. PM is good. Within scientific reason. I see no real good scientific reason for yearly bearing maintenance. Especially when no problem is indicated.