Do you need to file an extension if you do not owe any federal income tax?

Yes.

Technically, it is a crime punishable by up to 1 year in jail and up to $25,000 in fines to willfully not file on time. It is very improbable that you would be charged criminally, but it's out there.

Next, if you don't, you lose the option to make certain elections. Most of the elections are rather obscure things that the average taxpayer has never heard of. But there are a few, like the option to receive a return of your 2008 IRA contributions or to recharacterize a contribution until October 15th might be valuable.

And, finally, I don't know how many times I've seen people say "I get a refund every year there's nothing to worry about" and this would be the one year you wouldn't get one. If you haven't filled out your tax return yet, you don't know for sure whether you are getting one. And if the IRS audits you and finds that you are due a smaller refund or even that you have to pay, you'll have to pay an awful and unnecessary penalty for filing late.

The extension form is TINY. It takes just a minute to fill it out. You don't even have to send any money if you don't expect to owe any. Is there any good reason not to fill it out?