OK I guess this will sound weird to folks: I wear leather Sidies for both road and mountain biking. My feet get sweaty and will get wet while mountain biking (we have lots of stream crossing even in the summer). I have always done this and its doesn't seem to hurt the shoe at all. I run a sink full of warm water, add 2 capfuls of bleach and allow the shoes to soak for an hour or so (after first taking out the insole and soaking that separately). I then put them outside to dry in the sun. I also, after every ride, take a small spray bottle filled with alcohol and spritz out the inside of my shoes and helmet. It kills what ever bacteria and mold that can form and keeps my shoes and helmet smelling fresh.

There does come a time when shoes need to be replaced especially because they constantly get damp and used. I replace mine every 3 years or so...