wet felt changes its shape and thickness, letting your valves go out of alignment as well as making the valves sound "clunky". If they disintegrate in the casing, the valves don't go up and down anymore either. If you are playing a lot, once a month is not a bad idea for a bath! As far as the valves working fine, that is not a measure of anything. Even worn out valves with proper care move freely. Proper care keeps the valves TIGHT and moving freely!