NEW YORK (MONEY Magazine) - It's an accepted tenet of retirement planning: You need to stay invested in stocks -- keeping, say, 30 percent or more of your investments in equities -- if you want your savings to last the three decades or more you may need it for income once you stop working.

And that's sound advice, to be sure -- but not just for the reason experts usually give.

As it turns out, putting a substantial stake in stocks doesn't necessarily make your money last longer, but it does add mightily to your financial security. That's because the higher returns typical of a heavier stock mix help maintain your savings at a higher level, which gives you greater flexibility in spending and a bigger cushion against misfortune as you age.

Say you retire with a $500,000 portfolio, from which you plan to withdraw 4 percent to start and up that amount by 3 percent a year for inflation. As the chart shows, the chances that your savings will last 30 years are virtually the same whether you invest 20 percent of your money in stocks or 50 percent.

Pumping up the volume on equities, however, does make a big difference to the total amount of money you have. With half your assets in stocks, you have a 75 percent chance that your portfolio will be worth $190,000 after 30 years. With 20 percent in stocks, you're likely to have just $90,000.

One obvious advantage to having the larger stash is that you'll still have enough money to support yourself if your retirement exceeds 30 years. That's a distinct possibility if you retire early or live well beyond your life expectancy.

But there's another big benefit. A larger pool of assets amounts to an insurance policy of sorts that can, for instance, help pay your medical expenses later in life -- a huge plus given the rampant inflation in health-care costs over the past decade and the sharp drop in the number of employers offering retiree medical coverage.

A financial cushion can also reduce your chances of having to seek financial help from your kids or other family members -- a major fear among retirees, according to a survey from global bank HSBC.

And if nothing else, a nice reserve would give you some wiggle room if you ended up spending more in retirement than you'd anticipated, and could even allow you to indulge in the occasional splurge.

But beware: Investing too heavily in stocks can backfire. Since share prices can be decimated in severe or sustained market downturns, the chance that an all-stock or nearly-all-stock portfolio will last 30 or so years is actually lower than for a portfolio just half committed to stocks.

The right balance, according to Ned Notzon, chairman of T. Rowe Price's asset allocation committee, is to start off with 40 to 60 percent of your portfolio in stocks at retirement. Then, as you age, gradually ratchet down your stock holdings until they represent 20 to 30 percent of your assets by the time you're in your eighties.

In short, take the Goldilocks approach: not too light on stocks, but not too heavy either. That way your money should last a lifetime, with enough left to cover unforeseen expenses and, if you wish, to leave some dough to your undoubtedly deserving heirs as well.

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