When someone first becomes eligible for Medicare, they have the choice to sign up for part B. Part B of Medicare covers physician and outpatient services at 80% after a $144 deductible. Part B doe have a cost which is $104.90 a month. (deducted out of your social security check or billed quarterly)
Should everyone sign up for part B? The answer is no. Anyone that 65 or older and actively working may get stronger coverage from their employer. If they are actively at work, they do not need to sign up for part B. They can wait until they retire and sign up then. There will not be a penalty because they are actively at work. (same goes for someone getting benefits from an actively working spouse)
If you are getting benefits but not actively at work, you will be accumulating a penalty for not taking part B when you are eligible for it. The penalty is 10% of the premium for every year you are eligible but do not elect part B. This may be a problem if you ever lose your retiree coverage and need to sign up for B down the road.