If you’re single, not in great health, and are worth a lot but not a really huge lot, you could do your heirs a favor and die today or tomorrow. Sure, you may want to hang around to ring in the New Year but that could cost the beneficiaries of your will a chunk of change.

Wait a minute, you say, wouldn’t it be better to wait a few days and die in 2010 when, because the Senate didn’t act, the estate tax will disappear for a year and all inheritances will pass free of federal taxes? That’s true, as I discussed a couple of weeks back, but only if you’re really wealthy—worth more than $3.5 million ($7 million if you are married). If your estate totals between $1.3 million and $3.5 million, it’s cheaper—from a tax perspective—to die this year.

The little wrinkle causing that oddity is the 2010 limitation on “step-up in basis.” If you die before Jan. 1, anything you leave to your heirs gets a new basis for tax purposes, equal to the value of the asset when you die (or six months later if your executor chooses). If you’ve got assets with a lot of unrealized capital gains—think that IBM stock you bought back in the 1960s or that little cape you’ve lived in the past forty years—step-up could save your heirs a lot of capital gains tax when they sell.

Under 2010 rules, your estate may increase basis for  “only” $1.3 million of assets ($3 million if it goes to your spouse). The excess is stuck with your basis, called “carry-over.” That poses two problems: figuring out what the basis is and paying higher taxes.

Calculating basis is the first challenge. It isn’t a problem for stocks, bonds, and mutual funds acquired recently; your broker knows your basis for those. But what’s the basis for your house, which you bought in 1970 for $30,000 and on which you’ve made a variety of capital improvements over the decades? Or the family business? And that IBM stock you bought 40 years ago—it split how many times and you reinvested which dividends? A major reason step-up was created in the first place was to avoid those pesky valuation issues.

Let’s say your heirs manage to figure out your basis. Now, they get to pay the tax. Suppose your estate consists entirely of IBM stock—26,500 shares worth not quite $3.5 million. If you die today, your heirs would pay no estate tax on those shares and, thanks to stepped-up basis, they’d pay no capital gains tax either if they sold right away. (Of course, they would owe tax on any profits earned after they inherit the stock.) As a result, they get the full (nearly) $3.5 million.

If you hold on until Friday, there’s still no estate tax but your heirs enjoy stepped-up basis for only $1.3 million. That covers a bit more than 9,500 shares. But they’re stuck with your basis for the rest—about $2 a share for nearly 17,000 shares if you bought the stock in 1962 and reinvested all dividends since then. (Getting that $2 value wasn’t easy; I used simplifying assumptions and rounded off.) If the estate sells it all at today’s price, the federal tax on about $2.16 million of gains would total more than $300,000 and the states would get perhaps another $100,000. Is it really worth $400,000 (to your heirs) for you to live an extra day or two?

Of course, Congress might act retroactively in 2010 to re-establish the estate tax and reverse the carry-over basis rules so your heirs would never pay tax on your gains. But why take that chance? Die now and make your heirs happy. On the other hand, if you’re still having a good time, you might want to watch your back.