If you use machines as your main source of cardio, resist the urge to use the handles. They are there in case you need a little help with balance, but they should not be used as armrests, with you leaning your body weight into them. This inadvertently takes weight out of your lower body, making your workout easier on your legs. Plus, holding on to the handrails creates improper body mechanics, since most folks lean forward when holding on, and this bad posture can lead to injury.
If you find you can't use the treadmill, elliptical, or stair climber without using the handles for balance, then maybe you're going too fast. If you rest on the handles just because it's what you've always done, then there's no time like the present to break a bad habit. Instead of leaning on the handles, use those arms to burn more calories. Swing them from side to side or do bicep curls in the air. That way you'll be using your upper body too.