Many, many people are now working past the "usual" retirement age of 65. If you are one of these people but you have also reached your full retirement age (FRA) and want to begin receiving Social Security benefits, you need to be aware that you are not required to sign up for Part B of Medicare.
Part B of Medicare is the part that involves premiums. For 2012, the premium starts at $99.90 per month but the premium varies with income levels and some people pay as much as $319.70 per month. If you already have medical insurance through your employer, it is important to know that you can opt out of Medicare. However, the default is that the election (and associated premium deduction) is mandatory so you must specifically decline the coverage when you sign up for Social Security benefits.
It is also important to note that if you leave your job and your medical insurance ends, it is your responsibility to sign up for Medicare within 8 months. Failure to do so will result in the assessment of premium penalties and a possible delay in getting coverage.