It’s the best-kept secret in 401(k)s: free or low-cost professional investment advice. Three-quarters of 401(k) plans offer some form of help, from target-date funds and online tools to managed accounts. And taking advantage of this guidance can pay off, especially when it comes to reducing risk. For many workers, professional advice starts and stops with target-date funds, which simply shift your asset mix to be more conservative as you age. When you’re nearing retirement, though, you typically need more help than a single investment can provide. With most 401(k)s, you’ll find retirement calculators and tools; 39% also offer managed accounts. For a cost of 0.2% to 0.7% of assets a year (on top of investment fees), you’ll get a customized mix of your plan’s mutual funds geared to your goals and risk tolerance, either run by the plan’s investment provider or an outside adviser, such as Financial Engines, Guided Choice, or Merrill Lynch. Total assets in managed accounts, which tend to be held by pre-retirees, grew to $108 billion in 2012, up from $71 billion in 2010, according to Cerulli Associates. In 2012 workers using Guided Choice plan advice earned 2.1 percentage points more, with 50% less risk, than their colleagues who didn’t. Over the past five years, managed account returns lagged slightly, Vanguard data show. But, crucially, investors working with pros tended to be better diversified and saw steadier returns. Still, paying for advice isn’t right for everyone. Here are the three key times to do it: When you’re unsure where you stand. You can use your plan’s retirement calculator to check your progress. But you may find it more helpful to have a pro run projections, especially if you’re uncertain about what investment return, saving, and spending assumptions to make, or you need to take outside assets into account. When it’s time to trim risk. As you approach retirement, you need to shift to a safer allocation that will produce steady income. “The goal is to minimize the risk of a market crash just as you retire by creating an income cushion,” says Financial Engines CEO Jeff Maggioncalda. A target-date fund would give you that more conservative tilt. But with complicated finances that make diversifying difficult — such as company stock, outside IRAs, or a spouse’s plan — you’re a candidate for a managed account. Make sure the fee is reasonable — no more than 0.5% of assets. When you’re ready to retire. Both Financial Engines and Morningstar recently launched services that adjust your mix and calculate your withdrawals in retirement. If your 401(k) doesn’t offer this program (only 28% do) or you aren’t up to devising your own income strategy, hire an outside planner who charges by the hour or a percent of assets. You’ll also get help with taxes, insurance, and Social Security. After all, your 401(k) is only one piece of the retirement puzzle.