From my experience flying with them, I recommend you set up a frequent flyer number so you can put in a seating preference. They "try" to give you what you prefer (aisle vs window) if they have available what you prefer when they do seat assignments the day of departure. Before I had completed my preference, I was getting random seats, and ever since I put in a preference for aisle, I've always gotten it, and I never pay for a seat.