I don't need to tell you that I am pro cardio, and using cardio machines at the gym is a great way to get your heart rate up. However, you really need to avoid holding on to those handles. Not only is it bad for your posture and body mechanics since people tend to slouch when they use the handles, but hanging onto the rails actually reduces the total number of calories burned by 40 to 50 percent.
This is true for the treadmill as well as the stair-stepper. By not supporting yourself using the handles you are forcing your body to expend more energy and burn more calories. You are also forcing your core to work and support you as you move on the machine.
So let go! Or at least alternate every minute or two while you become accustomed to not holding the rails. If you feel a little unsteady read this post: Why Feel The "Y": Core Concepts. It will help you connect to your center and help your balance as you exercise.