I will use rubber from an inner tube only if the clamp isn't quite able to do the work on its own and needs more grip or a shim. I'm afraid that anything will trap and hold moisture inside and be worse than just bare chromed steel or aluminum. But I guess if you are worried about making a mark in super-thick glossy and clear-coated paint it's going to protect that a little bit. There seems to be a lot of differing opinions on the practice.