Many individuals do the exercise that way. The position itself isn’t necessarily the problem. What happens when you hold your arms out straight is that your other muscles will fatigue more quickly than your forearms. If you try to work through the fatigue and start leaning back, it creates poor form and unwanted stress. At the very least, your mind will start thinking about endurance and keeping your arms up instead of focusing on getting full extension with each and every roll. In essence, your wrist rolls will become increasingly less effective and the poor form could injure your back