Commercial mushrooms are grown in a substrate which contains sterilized manure. Any that I buy are going to take a nice long salt water bath and get thoroughly scrubbed down. A quick run through my salad spinner and everything's dry and clean. I haven't noticed any water absorption, and at least psychologically, the flavor's greatly enhanced. No BS! (couldn't resist).
Prep them the way you're comfortable with. I'm comfy with nice, clean mushrooms.