Many people don't clean their molds - they'll use the same one over and over for a given type of chooclate. Of course that only works if your temper is perfect each time, and if you're using the same colors each time.
I'm a fan of just using hot water. If your water is hard, you'll have to find a way to remove the minerals that are deposited behind - sometimes a cotton ball to buff it out works, but over time even that'll scratch up the molds.