It's important to note these facts:
"I often read in the financial press that taxpayers will pay no capital gains taxes if taxable income does not exceed the upper limit of the 15% tax bracket, which will be $73,800 in 2014 for joint returns and $36,900 for single returns. While this is correct, it omits the fact that those capital gains will be added to your taxable income, possibly pushing some or all of your gains out of the zero capital gains tax bracket."
"Let's say that Joe has $93,800 of combined annuity income, traditional IRA distributions, interest, and dividends and $20,000 of exemptions and deductions. His taxable income before selling stocks will be $73,800. Should Joe sell stocks with a long term capital gain of $10,000, all of the gain will be taxed at 15%, not zero. That's because his other $73,800 of taxable income has already pushed him out of the zero percent tax bracket before the capital gains are added."
"Retirees receiving Social Security benefits should also be aware that increasing AGI by selling capital assets for a gain might trigger or increase taxes on Social Security benefits. You may pay no Federal capital gains tax on the stock sale only to see your Social Security taxes increase."
The Retirement Cafe: Zero Capital Gains Tax