I would just like to add that the answer to this question depends upon the size of the business you work for. As per Federal guidelines, as a general rule, if your employer has fewer than 20 employees then Medicare is automatically the primary payer of claims and your employer-sponsored health plan is secondary. You do not have a choice in this.
Because of this, it is imperative that you sign up for Part B as soon as you become eligible. (Part A covers facility charges, Part B covers physician charges.) IF YOU DON'T, YOUR EMPLOYER'S INSURANCE POLICY MAY DENY PAYMENT FOR ANY PART OF YOUR CLAIM THAT WOULD HAVE BEEN PAID BY PART B! AND, you may have missed your opportunity to sign up for Part B and could have to wait til the open enrollment period (Jan 1 to March 31 each year), and your coverage will be effective the following July 1, leaving you with a gap in coverage for Part B services.
If you work for a larger employer (generally 20 or more employees), then your employer-sponsored health plan will be primary and you do not need Part B. This rule is called Medicare as Secondary Payer and is part of a Federal law called TEFRA/DEFRA.
As a licensed health insurance broker I see this all the time. "I didn't sign up for Part B because I have insurance through my job."
Check with your employer's Human Resources Dept or your employer's health insurance broker and ask if your employer is subject to TEFRA (Medicare as Secondary Payer) rules, and/or call your local Social Security office before making a decision about Part B.
And heads up, I've seen situations where the SS representative failed to ask the individual about the size of their employer, resulting in misinformation and no coverage!!