I am retired CSRS also and I have Part B and kept my FEHB plan (NALC). If you don't apply by the end of Oct (I think) for Medicare, and, if there is not a change to current law, your premium for 2016 will be over $150 vs the $105 now. That's because of a hold harmless rule that prevents Medicare payments from going up for about 80% of current recipients when there is no SS increase, so all the increase in program costs are dumped on the remaining 20%. Sorry, I know this is confusing, but Google "Step Increase in Medicare Premiums for 2016."
Also, the decision depends on your health, whether you are married, and if your CSRS annuity plus any other income you make will exceed the level where income based premiums kick in. Also, if you are not collecting SS and have your premium deducted from it, you also fall in the new, higher premium for 2016 as folks in that category are not eligible for the hold harmless provision.
It is a very complex decision for retired feds especially CSRS. You have a 7 month window to apply - 3 months prior to turning 65, the months you turn 65 and 3 months after. If you go beyond that, you can end up paying a 10% annual penalty. Whatever you do - DO NOT drop your FEHB - you can never get it back once you retire. You can take Medicare and then drop it in the future. You can even get it back again, but the 10% annual penalty applies. You probably don't collect SS yet ( and as CSRS you may never), but, if you have it, they will automatically enroll you in the month of your 65th birthday unless you tell them not to.
Lots of things to consider. Plus, if you do take Medicare, you can switch to a lower cost FEHB plan with just basic benefits this open season. Best of luck.