Q: I’m a Federal Employees Retirement System employee and I am confused about what advantages there are to the payroll tax deduction reducing the contribution into Social Security from 6.2 percent to 4.2 percent in 2011 as part of a payroll tax holiday. Won’t I still need to pay 6.2 percent at the end of the year when I’m filing my taxes? And will it not also affect my Social Security?    A: No, you won’t have to pay the difference at the end of the year. Right now, Social Security benefits are not indexed to the rate at which you…