NEW YORK (TheStreet) -- Disciplined investors know it's important to stick to the plan. If a stock surge leaves your portfolio with 65% stocks instead of the 60% you'd intended, well, it's time to sell some stocks and move the proceeds to bonds or cash.

At least, that's the standard advice. But stocks have kept climbing this year, surprising many who thought last year's stupendous gains would be too tough an act to follow. Should you just skip that midyear rebalancing to keep riding the stock market wave?

First consider your investing time horizon. If you're investing for a retirement that won't start for 20, 30 or 40 years, a short-term tactical move such as delaying rebalancing probably won't make much difference. Asset allocation strategies are based on long-term market patterns that tend to smooth out short-term ups and downs. And lots of research shows that even the pros aren't very good at market timing.

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Second, what's the opportunity cost of, say, keeping a higher-than-planned portion of the portfolio in a given asset class? Opportunity cost is the price you pay by not moving your money into that other asset. If you stay in stocks and bonds do better, you'll miss some gains. 

Granted, bonds don't look very appealing right now, because rising interest rates could undermine bond prices. But even if bond returns are weak, they could beat stocks if investors start to get anxious about high stock prices, causing them to flee the stock market and drive prices down.

Third, be realistic. Keeping 65% of your holdings in stocks instead of 60% probably would not have a dramatic effect on your returns, even if stocks did well. If stocks gained 10% and bonds were flat, a $100,000 portfolio with 65% in stocks, 35% in bonds, would grow to $106,500, while the 60/40 portfolio would grow to $106,000.

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Fourth, how will you get your plan back on track? Before committing to a higher-than-planned stock allocation, it would be wise to set some rules for reversing course. You could set a deadline or vow to pull back on stocks if the ratio between share prices and corporate earnings rises to a given level. Or you could put a larger portion of new investable cash into bonds to get back to the desired portfolio balance over time. 

Finally, think about how you'll feel if things go wrong. If you keep more of your holdings in stocks and they plunge, would your long-term plan fall apart? Would you hate yourself? Or could you shrug it off as a lesson learned? 

Investors have every right to tinker with their long-term plans as conditions change. After all, asset allocation strategies are based on past patterns that won't necessarily be repeated. But short-term gambles do have a way of backfiring.