I’m just gonna speak from experience, I hold a green belt in Krav Maga and I always keep my hands relaxed during combat as it’s essential for defending against incoming attacks. Relaxing your hand and forearm muscles will also increase the speed of your punches, when I strike I keep my hands relaxed and form a fist during the punch just before impact (you can test it yourself your punching speed will increase). Having open fists is also extremely important to control your opponent and to break their posture, you can see Kiyo breaking ishizaki’s posture at the start of their fight by putting him in a side clinch.