Nope. We use 100% of it, but not always simultaneously (due to its incredible resource requirements). This is completely supported by fMRI (functional magentic resonance imaging) among other methods which demonstrate brain usage, many animal studies including studies done on humans, and evolutionary common sense: the brain is the body's most intricate, highly-protected and energy-hungry organ. Using only 10% of it despite its extraordinary energy demands and intricacy is evolutionary suicide.