My elder gelding LOVES having his sheath cleaned. If I don't keep up with it, he'll drop as if to say "Hey, Mom. This thing is looking pretty bad." The best sheath cleaner I have found is from E-Z All. I've just started cleaning my two youngest geldings, and they are actually being quite good about it. I look at it this way, keeping it clean is probably going to help decrease the chance of infection and irritation. Especially since those darn gnats swarm on a dirty sheath.
"My riches do not clank and glitter. They gleam in the sun and whinny in the night."