But many experts question whether a mix of 60 percent stocks and 40 percent bonds is suitable. Over the years, 60/40 has become a rough gauge for how to build adequate retirement savings without taking excessive risk. Typically, it's promoted as a sort of default portfolio balance for investors in their 40s, or even closer to retirement.

The basic rationale: Keep most of your retirement savings in the stock market, because stocks are likely to provide greater long-term growth than bonds. But when stocks fall, you'll want a significant percentage of your portfolio in bonds to cushion against steep losses.

That approach is under fire. Many suggest it's important to have more than 60 percent invested in stocks. That's because retirement can stretch for several decades, and investors will need to increasingly rely on stocks to limit the risk of outliving savings. Also, the long-term outlook for bonds is poor. Their yields are near all-time lows and interest rates are certain to eventually climb.