If you think you might not be able to file your tax return by the April 15th deadline, you should absolutely file an extension. That gives you protection against late filing penalties if you are unable to file by April 15th.

An extension, however, does not protect against interest or late payment penalties. You still need to pay the IRS any taxes owed by the April 15th deadline even if you file your tax return after that date. However, late filing penalties are a lot larger than interest or late payment penalties, so an extension should be filed to protect against those penalties. You can file a free extension on FREETAXUSA.