Two Problems with Discounting Tax Impacts on Social Security Benefits
by Rich White
First, it is possible and perhaps likely that the taxable portion could increase to 100% in the future as a politically attractive way to strengthen the system's fiscal status. Secondly, because benefits are paid monthly, they offer little tax-planning flexibility, unlike many other sources of retirement income such as Traditional IRA withdrawals prior to age 70.5.
Source: http://www.investopedia.com/articles/retirement/08/6-retirement-myths.asp