This depends on which part of the fed employs you. TVA, for example, does recognize unused sick time at retirement, though not in a 1:1 relationship used towards service time. But there is a benefit to NOT using sick time there. It's a useless benefit if you have a major illness and need to take the time off.
I lost over 250 days of accumulated sick time when I left the private sector 5 years ago. No compensation, no nothing.
There's a fair amount of variance between the various federal agencies.
Best thing about sick leave, though, is NOT HAVING TO USE IT.