If you are receiving Social Security benefits, as rjs said the increase in taxable income caused by the inclusion of the capital gain can make more of the Social Security benefits taxable. This side effect would make your marginal tax rate on the capital gain be nonzero even though the tax rate on the capital gain itself is zero. You'll need to prepare a simulated tax return to see what change in tax liability is caused by a change in capital gains income.
You also need to check the marginal tax rate on your state income tax return.
