Open hand is where it's at. I never learned closed fist with over hand serves. You have a lot more control with an open hand. You can do float, spinners and you can direct where you want the ball to go. You want more surface area. A closed fist limits that and also if your fist is not flat it will make your serve unpredictable. As you move up in levels the serve becomes very important. It's your first offensive attack so you need to make it count.