According to Fidelity, the number of 401(k) millionaires has recently reached a new record.

Not coincidently, the U.S. stock market, as measured by the S&P 500, is also at an all-time high. If you've participated in this prosperity, I congratulate you.

If you have seen your retirement savings blossom and are closing in on retirement, investing in bonds is an increasingly good idea. That's because the closer you are to retirement, the more difficult it becomes to replace lost retirement assets with future savings.

The cold hard reality is: You never know when the next big stock market sell-off is going to happen. The good news is that understanding the benefits and risks of owning bonds and the role they can play in your retirement portfolio can help make your retirement savings more secure as you near retirement.

Historical risk and return

A good way to understand the risk and reward of owning bonds is to compare them to stocks. While both are financial assets a corporation or government issues, purchasing a share of stock represents taking an ownership interest while buying a bond represents making a loan to a company or government entity.

Through the past 60 years, stocks have delivered an average annual return of 10.7%, while bonds have generated an average yield of 5.8%, as measured by the 10-year Treasury. During that same time period, the risk, or volatility, has been on average about 60% higher owning stocks with a "worst-case" decline of around 50% for stocks compared to a 15% drawdown for bonds.

Current valuation

Like any asset, your expected future return depends partly on what you pay. From this perspective, stocks look fairly expensive, trading at 21½ times future earnings, which is more expensive than they've been 91% of the time in the past 20 years. Bonds yield about 4%, as measured by the 10-year Treasury, which is a higher yield than 84% of the time in the past couple of decades. While stocks are certainly able to go higher for various reasons, allocating more assets into bonds looks relatively attractive right now.

Risks for bonds

The primary risk to owning bonds is rising interest rates. When interest rates go up, the value of existing bonds and bond funds go down. Importantly, if you own individual bonds, you normally can't lose money if you hold them to maturity. Central banks like the Federal Reserve adjust short-term interest rates to control inflation and stimulate economic activity. U.S. government spending and taxation policies affect longer-term interest rates, primarily because they determine how much money the U.S. government must borrow.

Foreigners own about 30% of all U.S. bonds, so foreign demand also plays an important role in our bond market. As long as the U.S. remains a military and economic superpower, there should be ample foreign demand for our government debt, which keeps our interest rates lower. Any government that borrows too much threatens its ability to repay the principal and interest on its debt, which would dissuade foreigners from future purchases, causing prices to fall and interest rates to spike.

Investment strategy

If you sit down with a pro and ask about your proper allocation to bonds, the conversation will invariably start with risk tolerance. How much do you need to have accumulated to meet your financial goals? How much time do you have to make up a big loss before you want to start spending your savings? Without such an investigation, the historical rate of return data I've already shared suggests everybody would invest 100% in the stock market.

Since now is one of the better times to invest in bonds in the past 20 years, you might be asking, What's the best way to get started? For anyone older than 50, I'd start with a 10% position and move 5% more into bonds every year until you're at a 50-50 split. The most conservative allocation I generally recommend is a 70% allocation to bonds. Any more than that carries with it the very real risk of losing purchasing power because of inflation.

It's also important to understand and practice the discipline of rebalancing. I recommend revisiting your portfolio's stock/bond mix once a year. The next time stocks suffer a steep decline, sell some of your overweight bond exposure and buy more stocks to rebalance back to the original target weight. Practicing this discipline will force you to buy low and can meaningfully enhance your overall portfolio's long-term rate of return.

Ultimately, a careful balancing of the need to secure hard-earned savings and the desire to grow your retirement nest egg should guide your ideal exposure to bonds. If in doubt, find an experienced financial adviser, preferably one who will work for an hourly rate, to provide guidance.

Michael J. Francis, is president of Francis LLC, a registered investment adviser with offices in Brookfield, Wis. and Minneapolis. Mike Francis can be reached at michael.francis@francisway.com. The information contained herein is provided for informational purposes only.