(MONEY Magazine) – Q I donate my family's used clothing to a local charity's resale store. The store gives me a tax receipt that shows the number of boxes I've dropped off, then I estimate the value of the clothes. I've always made a conservative estimate, but my brother-in-law Alex says I'm nuts. He thinks I should write off $200 to $250 a box since "no one knows what's in it" and "everyone inflates charitable contributions on their taxes." Is he right?

A Well, he's correct that no one knows what's in the boxes. And he's also right when he implies that this form of fiddling is unlikely to be detected. But he's foolish to insist it's not wrong to write off more than the value of the items you're donating. And you are fooling yourself if you believe him.

Alex may have convinced himself that since it's impossible to put an exact value on used clothes, he's only being aggressive. But knowingly overvaluing your donation isn't being aggressive. It's being dishonest—a tax strategy that amounts to nothing less than cheating on the honor system. Sure, some people do it. But most taxpayers don't: Myriad studies have shown that the majority of Americans do not cheat on their taxes.

The real question, though, is how important it is to you to do the right thing when it comes to filling out your income tax return. If it's important, you shouldn't care how many other taxpayers take the high road with you or how likely you are to get caught by the government if you don't. Ethics, as the saying goes, is what you do when no one is looking.

Questions about money and ethics? Our ethicists are consultants who advise attorneys on people's ethical beliefs. E-mail them at right_thing@moneymail.com.