Do I need to rehydrate mushrooms before I cook them?
I'm guessing you're referring to dried mushrooms, right? Yes, unless you want to mimic the chewing on shoe leather experience. Soak them in very, very hot tap water to cover. Put a plate or something flat over the bowl so the heat is retained. Wait 15 minute. Strain them, and keep the soaking water - it's loaded with rich flavor and color.
I have no idea why you got the answer twice. With the same typo on 15 minute.
I learned a great trick from a friend from Tuscany. If you are using dried porcinis, soak them in cream and keep the (then) flavored cream for use in your recipe.
Also, if you are using dried shiitakes, those take a little longer. Best bet is to soak overnight in the fridge. A trick I have (no patience) is to put the dried mushrooms in the blender, turn it into a powder, and use it to thicken/flavor sauce. Instant gratification.
Make sure to watch out for the grit/silt that descends to the bottom of the bowl. I forgot once and dumped the whole lot into my pot roast and the sauce a bit crunchy.