It's often wise to roll the retirement plans you accumulate during your career into an individual retirement account. But in some cases, it can be a big mistake with unpleasant tax implications.

Baby-boomers approaching retirement are likely to move more of their nest-eggs into IRA rollovers, so financial services firms and advisers want a piece of that asset-filled pie: Assets rolled over into IRAs from employer-sponsored plans like 401(k)s were estimated to hit $194 billion in 2005.

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That figure will rise incrementally each year, and is expected to reach $387 billion in 2010, for a grand total of $1.684 trillion over that five-year period, according to Cerulli Associates, a Boston-based research and consulting firm.

While rollovers have advantages for many workers, there are certain issues investors need to be aware of before making any decisions.

The Internal Revenue Service recently increased the fee — to about $3,000 from about $95 — to seek a ruling to correct a rollover gone awry.

"Rollover sounds easy, but it's difficult and a lot of people end up losing a big chunk of money in the process," said Ed Slott, an accountant and IRA consultant based in Rockville Centre, N.Y. "Many of the mistakes are irreversible."

One of those mistakes involves individuals who hold highly appreciated company stock inside their company-sponsored plans.

If someone in this situation rolled their shares over into an IRA, "you just signed yourself up for an incredible tax bill that you didn't need to sign up for," said Rob Kron, a director within the retirement group at Merrill Lynch.

Here's why: a rollover would preclude the investor from executing a tax-saving strategy known as "net unrealized appreciation."

This allows the investor to withdraw these shares, put them in a taxable account, and pay ordinary income taxes only on the cost of the stock when they were first acquired. Any gains would be taxed at the much lower long-term capital gains rate of 15 percent — rather than ordinary income tax up to 35 percent — when the stock was sold.

Another specific scenario concerns certain people born before 1936 who are eligible for 10-year income averaging. This is a once-in-a-lifetime election that allows you to pay tax on a lump-sum distribution as if the amount had been received over 10 years, rather than all at once.

The tax on the averaged amount can be figured at a lower rate, and usually results in smaller tax bill, according to Smith Barney's IRA Rollover Guide.

However, you'll still have to pay that tax bill when the distribution is received; in this case, the whole amount must be withdrawn and taxed.

This strategy may not be suitable for everyone, so you'll need to consult with a tax adviser.

Other reasons to stay within your company plan: if you're content with its investment option menu — despite the ability to choose from a vastly greater universe with an IRA — it's likely that investments within a big plan cost less, too.

Also, you can typically borrow from a company plan but not from an IRA. And distributions taken from IRAs before the age of 59.50 are subject to a 10 percent penalty, whereas there are no penalties within company plans for individuals age 55 or older that have left that employer.

Despite these and other caveats, there are also many advantages to IRA rollovers. Besides providing you with a vastly wider array of investment options, IRAs are also more flexible for estate-planning purposes.

If you or your adviser decide that a rollover is the best course of action, be sure you know the rules to avoid unintended tax consequences.