Some myths about EXTENSIONS:
1. "If you want to lessen the chance of an audit, file right at April 15. They're so busy then, they don't really have time to look closely at an audit. Or, better yet, file an extension and file later, when they have already chosen all of the audits for the year."
FALSE. Filing at the deadline or an extension does NOT make you less likely to be audited. Audit criteria is screened solely by computer these days, and the program takes the same time to screen returns every day of the year.
2. "If you owe, you can file an extension to pay it six months later."
FALSE. In filing an extension, you are required to make an estimate of what you owe, and pay it with the extension (Form 4868). Anything you pay after April 15th is subject to interest charges. In addition, if it is in excess of $1,000 AND 10% of your total tax for the year, it is subject to a half percent per MONTH (or part of a month) penalty for late payment.
3. "If I forget and don't file my extension by April 15, I can file it late and just pay interest or penalties for those few days I missed."
FALSE. The IRS will only honor an extension request if filed on time. Otherwise, your return is simply late, and subject to interest and penalties as indicated above.
However, be aware that the IRS is waiving some late filing/payment penalties this year, for those who have to use forms that were not available until early March (such as the passive activity loss form, educational credits, energy credit, etc.) See IRS.GOV for full info.
4. "Since late filing penalties are based on a balance due, if you don't owe anything, you can file late with no negative effects."
FALSE. While it is true that penalties and interest are only assessed on a balance due, failing to file on time can limit the availability of certain elections, deductions and credits, which may only be available on "timely filed returns." While not likely to affect most filers, it is something to consider. Furthermore, intentionally NOT filing a timely return is technically a violation of federal law, and can be prosecuted ... though this is highly unlikely to happen, unless you are caught by the IRS many years later ... or you decide to run for public office. :)
Extension form is at link below.
Note: Each state has its own rules. Some (like my home state of AZ) accept a federal extension as applying to state as well, although they do require payment of any tax due. CA grants an "automatic" extension (whether or not you extend federal), but wants the balance due paid by 4/15. Check your local state's rules.