After I wash mushrooms in salt water, I dry them in a salad spinner. Then I insert the spinned mushrooms into my wok, that's hot with safflower oil. At this point, the wok has no water. After 3 mins. of medium heat, the mushrooms release water. Is this water useful? Are there safety concerns with it? Or can I use it for stock? I'm guessing that it's safer to dispose it, because this water probably hails from the growth and cultivation of the mushrooms, and we can't know what quality of water they use. Some of the water may hail from the tap water + salt that I cleaned the mushrooms with. 
The water from cooking mushrooms is good for flavour. When making vegetarian pasta sauces, I often add mushrooms and their cooking water to the sauce, for example. These get well-fried, but not typically until the water evaporates, which of course concentrates the flavour - that's still good. The only time I waste it (and it does feel like a waste, perhaps I should freeze it for pasta sauce) is when I cook them to add to an omelette, either by frying or in the microwave.  That's because I don't want the water in the omelette. I do lightly rinse mine before use, but let them dry for a few minutes so they add very little water to the pan.  I find this more effective in removing dirt than wiping, and don't have a soft enough brush. As an aside, plants are quite good at turning dirty water into clean juice (and sap etc.).  Mushrooms aren't plants, but I would expect something similar.  Anyway, they're usually grown under very controlled conditions, so the water isn't likely to be contaminated. 