Um... hello... knee pads? Especially for Iaido!
I did use knee pads when first starting out, but the school I train involves a lot of cutting down to one knee from a standing position. In this case, knee pads can actually be bad, because you don't learn the proper feeling for bringing the knee to the floor, and end up bringing it down much harder than necessary, putting more stress on it. Taking off the kneepads improved my iai tremendously, and greatly reduced post-keiko knee soreness.