Should I clean fresh mushrooms before eating or cooking them?

 
It’s a good idea to go over them with a very small amount of water.  You can use a soft brush to remove any traces of peat moss or soil.  They can also be gently wiped with a damp cloth. If you’re concerned about chemicals (fertilizers), wash them gently and quickly under running water.   Pay special attention to the underside of the mushroom because the flesh under its cap can store bacteria and viruses.  But don’t overdo it.  Cleaning vigorously can cut their skins and push bacteria and spores into the tissue. 
 
After washing, mushrooms should  be patted dry because they are very porous and will absorb water easily. They should never  be soaked in water unless you like the taste of soggy mushrooms.  Before cooking or using them raw in a salad, cut off any dark spots and trim the ends of the stems. 
 
  
Source(s):
Mushroom Council "Care and Handling for Customers"
Health Canada "Safe Handling of Mushrooms"
Suite101.com. “Mushroom Facts and Guidelines A Simple Guide for Buying, Storing, and Preparin Mushrooms” 
About.com.  “Home Cooking: How to Clean Mushrooms.”