I think this is a really personal decision. At a SWA interview, they want to get to know you. If they feel you are putting up shields and spouting memorized answers, you aren’t getting hired. By the same logic, if you are too nervous to speak coherently, they won’t get to know you either and you won’t get hired.
You need to walk into the interview confidant enough to talk about yourself and having examined your personal experience enough to show that your background, actions, and attitudes will serve the company well.
Having never interviewed before and knowing how nervous I’d be walking into my first interview for a multi million dollar career, I chose to use a prep service to help me get some interview skills. It helped me look back over my experiences with a different perspective and realize what I bring to the table. It let me walk in still nervous, but with enough confidence I could relax a bit to “be myself”, speak coherently, and land the job.
A lot of folks can do that without prep. You may or may not be one of them.
They didn’t ask, but I was ready to explain why I thought it was important.
I interviewed spring of 2017, in case the landscape has changed since then. Hope that helps.