Will that work? 'The internet' (e.g. http://news.cnet.com/8301-13579_3-57602245-37/iphone-5s-come...) claims the sensor uses capacitance, not a camera, so you might need tinfoil tape.
Well. Being cynical for a moment, that's a great way for Apple to guarantee the fingerprint scanner always gets a good capture.
If I ever own one of these phones, I might have to make it a point in my muscle memory to use my knuckle to press the home button.
It's your hardware after all. If you're worried about it, take steps to protect yourself.