They might be the most recent addition to your 401(k) plan, but that's no reason to take your eye off your target-date fund line-up.

"Are You in the Wrong Target Date Fund?" is a new white paper from Towers Watson that makes that point, and others, about TDFs. Why? So much money is going into them, and there's so much change in the funds themselves, that even TDFs added only a few years ago might already be obsolete from a suitability standpoint, according to TW.

Lest you doubt its importance, keep in mind that, according to the white paper, 86 percent of direct contribution plans now use TDFs as their default investment option. And employees are taking advantage. The Employee Benefits Research Institute says that TDF assets within 401(k)s tripled between 2006 and 2012, from 5 percent to 15 percent of aggregate assets. During the same time period, participants using TDFs more than doubled, from 19 percent in 2006 to 41 percent in 2012.