Do I need to file an extension if I'm getting a refund?

Technically no, at least when it comes to your federal return.

Because the IRS calculates the late-filing penalty off of any additional taxes you owe ($0 in your case), there's no penalty because $0 × anything = $0.

However, it's best to go ahead and file an extension anyway, for three good reasons:

1. If you end up owing (for example, if you corrected an error, forgot to enter a W-2, got a revised 1099 form, etc.) an extension protects you from the late-filing penalty.
2. Some states require that you file a federal extension to get a state extension.
3. It only takes a few minutes, and it's free!