.tim said:
If you tithed on the original income, you don’t need to tithe again ... right?
It depends; for people just retiring today, probably not.
Consider: if you went to the store, and bought $30 of groceries, and handled the clerk a $50 bill, you'd get $20 in change. No one that I've ever heard preach claims that you should "tithe" on the $20.
In the same vein, if you purchase something for $150 and you receive a refund of $30, I don't consider that $30 an "increase" - it's no different (to me) that getting change in the first illustration. I have to confess that I have heard at least one "preacher" claim that since the "regular" price is $150, you saved $30, thus having an "increase" of $30 and one should tithe on it. (I don't agree with this rationale)
When we come to social security, it's a forced savings program (in theory); while you were working, the government was taking your money, holding it for you, and then giving it back after you retire. Thus, if you contributed $100,000 to SS over your working lifetime, I would not tithe off of the social security income until and unless it exceeds what I put into it; before that, they are just paying you back your own money.
Note that the above is a very tightly defined and highly rules-oriented look at what to give - it's a bit like the Pharisees counting mustard seeds and anise to get exactly the required offering. Jesus said, talking of giving,
For with the same measure that ye mete withal it shall be measured to you again.
Bottom line: I would consider, depending on my circumstances, giving some of it as income -- I would ask God to provide guidance, and be willing to follow him, but I don't believe a tithe is required based on the discussion above.
Final note: I know that many do not believe that a Christian should tithe; I am NOT addressing the issue here; I assume that you believe in tithing, and I have answered accordingly. I personally believe that a Christian should support his church and should ask God what he should give; it may be much higher than just 10%.