JOEL - Everyone who wishes to remain in good standing in the Church and be temple recommend worthy should declare their tithing status to the Bishop, no matter what amount of tithing they pay; even if it's nothing. Tithing settlement also gives the Bishop a chance to visit with his members to personally see how they are doing.
You might want to talk to your Bishop for your particular situation as far as tithing on Social Security Income is concerned. You might think you already paid tithing for the Social Security money that was withheld from your paycheck if you paid according to your gross income. But if you think about it, the Social Security money you receive back after you retire is probably going to be far greater than anything you had withheld from your paychecks.
But ultimately it is up to you, what you decide is an honest full tithe. A 1970 letter from the First Presidency stated that notwithstanding the fact that members should pay one-tenth of their income, "every member of the Church is entitled to make his own decision as to what he thinks he owes the Lord and to make payment accordingly" (Doxey, Roy W. Tithing: The Lord's Law. Salt Lake City, 1976).