It's a debate that has dragged on for years and now BlackRock has issued a white paper in which it officially weighs in: "to" target-date funds are preferable to "through" TDFs, it says.

"It is BlackRock's position that a persuasive common-sense case can be made for the 'to fund' approach based on an understanding of human capital, or the ability to earn income, which is depleted at retirement, and retirement risk, which we argue is at its highest level the day retirement begins," wrote Ted Daverman, vice president, U.S. Retirement Group, and Matthew O'Hara, managing director, U.S. Retirement Group.

"To" funds, of course, are those with a managed roll down of equity only until the day of retirement, after which the asset allocation remains static, while the equity allocation of "through" funds continues to diminish beyond the target date.