Harold McGee, the guy who wrote “On Food and Cooking” and “The Curious Cook” (the best books to date on the science behind cooking) said the following in answer to this question:
“I was skeptical about the mushrooms-absorb-water idea and so did the soaking experiments with standard white mushrooms for “The Curious Cook” back in 1990. I’ve since tried a number of others, and if you make sure to shake the water out of the nooks, fresh mushrooms absorb little if any water. I’d also say that since they’re already around 90% water, a little more or less isn’t going to make much of a practical difference in the subsequent cooking.
Now losing flavor to the water is a different question, and it’s possible that you might lose savory amino acids (aroma molecules tend not to be soluble in water). Even here, though, the amino acids are inside cells, and most cells are not going to be breached by surface exposure to water.
So I wash my mushrooms with a clear conscience”