A lot of guys are saying the right thing here.
It really is to stay loose and relaxed. This is because when you throw a relaxed shot, you throw it faster, you are able to snap your punches and helps get a feel for the "whip" like the effect that your punches are supposed to have. You can accomplish the same things with a closed fist, but it takes less focus to do it when your hands are loose and not tightened. This allows you to then get a feel for what a snap feels like. So when you throw with the fist later, it will be the equivalent of swinging a kettle bell with a chain.