No, you don't need to temper chocolate for sauces, mousses, cakes, or other desserts.
Tempering chocolate is for molded chocolates and showpieces. It basically keeps the cocoa butter from separating (blooming) to the surface of the chocolate, by keeping all the molecules in place. If you were to make molded chocolates without tempering, they'd likely stick to the mold, break easily, and eventually develop a whitish sheen on the surface. Not very pretty.