It is not only a common courtesy to wipe down machines and equipment after using them, it is a posted rule at many gyms. I am not sure about your gym, but I am fairly certain that most of my fellow gymmates are not pulling their weight (pun intended) and cleaning up after themselves. Have you ever thought of wiping them off before you use them? I am a germ-a-phobe and with flu season raging on, I have started to wipe off equipment, focusing on handles and buttons, before I use it. It may be a little anal, but it's better than catching a nasty bug that someone else has.