Selling securities at a loss can cut taxes. But it might not be for everyone.

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So, should you consider swapping out mutual funds, exchange-traded funds or stocks whose prices have dropped to soften next year’s tax bill?

Some financial advisers and investing experts offer a word of caution. Holding on to investments purchased to fulfill a long-term strategy means taking the bad years with the good, they say. Unless a portfolio is being overhauled for other reasons, they say, shifting assets around to gain tax benefits can distract investors from the big picture, potentially knocking them off the path to achieving their long-term goals.

“You need to find a benefit to doing this aside from just your tax bill,” says Christine Benz, director of personal finance at fund trackers Morningstar Inc.

Harvesting in place

Some proponents of tax-loss harvesting point out that it doesn’t have to involve any long-term shifts in a portfolio. If you like a particular security but its value has dropped below the price you paid for it, you can sell it to take advantage of tax-loss harvesting and then repurchase it—as long as you wait 30 days between the sale and repurchase, to satisfy “wash sale” rules, notes Ashley Murphy, a certified financial planner with Areté Wealth Strategists in Minneapolis.

“You can park the funds from the sale in an ETF that has similar properties, then return to the original fund or stock,” says Mr. Murphy, who says he typically harvests tax losses for two-thirds of his eligible clients each year.

But Kent Grealish, a fee-only certified financial planner with Grealish Investment Counseling in San Bruno, Calif. warns that selling and then reacquiring assets this way can be costly.

“You’d pay an adviser or CPA to help you do this, plus transaction costs, plus you’d be exposed to the market for 31 days,” Mr. Grealish says. “To me it’s not worth it. It doesn’t really live up to its promises.”

When to harvest

Proponents say a good time to consider tax-loss harvesting is whenever portfolio shifts are already under discussion—which doesn’t have to be a year-end conversation.

“Most people look at tax-loss harvesting at the end of the year only, but this is a huge mistake,” says Chris Hardy, a certified financial planner with Paramount Investment Advisors in Atlanta. “Throughout the year, there are times when a person’s portfolio could be down and they could take advantage of this temporary situation by selling the loss and purchasing a similar index fund or ETF as a replacement during the 30-day wash-sale period. This strategy is especially good for those in the 25% tax bracket or above.”

Mr. Murphy, the Minneapolis adviser, says he recently harvested a tax loss for a client whose portfolio held an emerging-markets ETF that Mr. Murphy was considering replacing and that had dropped in value. In looking at other options, he found a lower-cost Vanguard emerging-markets ETF with a lower expense ratio, and so he shifted the client there. He didn’t significantly shift the client’s portfolio allocation to emerging markets—he just found a cheaper, better-performing ETF and took advantage of a tax perk when reallocating the client’s funds.

Other options

Tax-loss harvesting needs to happen by the end of the calendar year to apply to that year’s taxes, so it’s too late for investors looking to reduce their tax bill this April. One investment option for them, if they’re eligible. is tax-deductible contributions to individual retirement accounts. Contributions made by April 15 of this year can be applied to 2014.

Tax-loss harvesting also can’t happen without losses, of course. But, looking ahead to next year’s tax bill and beyond, some investors whose holdings have done well can consider a tactic called tax-gain harvesting.

This makes the most sense for investors in the 10% to 15% tax bracket, who pay no taxes on capital gains. For these investors, it might make sense to sell some investments to realize the gains tax-free. The money can then be immediately reinvested any way the investor wants, even in the same security or fund that was sold—the wash-sale rule doesn’t come into play in this case.

The idea here is to take gains before you move up to a tax bracket where you will be subject to capital-gains taxes. However, there are several potential pitfalls, so anyone considering this approach should consult an accountant or financial adviser or look for detailed discussions of tax-gain harvesting online.

Ms. Hodges is a writer in Seattle. She can be reached at reports@wsj.com .