I think one of the theories as to how spinach got contaminated was that farmers were allowing their livestock to wander in the fields and their shit was what caused it all. Still that doesn't explain the tomatoes, since they don't grow on the ground.

You can wash muchrooms right before you eat them, it won't hurt them, but you definately don't want to put them away wet, unless you're trying to make homegrown penicillin. ;)