No, you don't need to file an extension if you don't owe the IRS any money. You only need to file an extension if you need more time to pay.
The current law is you have three years to file for a refund and after that you forfeit it to the IRS. The IRS does not charge penalties to people that are owed refunds even if they are filing after due date (usually April 15). Just make sure you file within the 3 years.
But if you owe money, even if you file an extension, if you do NOT pay by April 15, then you will owe interest and penalties on the money you owe. The extension is form 4868 and can be filed online through IRS.gov e-file, or by mailing in the form. The extension is NOT an extension for time to PAY, it is just an extension for time to file.
Even if you are owed a refund, the IRS says that you are "required" to file if you made over the standard deduction. But in all reality, if they owe you money, they don't want to see you get it. If they owe you money, they don't care when or if you file, but they do like to keep tabs on you. There will be no penalties.