It is perfectly acceptable to file an extension with the IRS... there is no reason not to if you find it necessary to do so. However, you MUST pay your estimated tax due PRIOR to the regular tax deadline in order to AVOID any late penalties and interest. If you cannot pay the full amount you owe, pay as much as you possibly can with the extension form. If you own a business, sometimes filing an extension can be to your advantage if it allows you more time to search your records for additional deductions; however, you still need to pay as much as you can with the extension.
The extension deadline is October 15 (unless it falls on a weekend).