When I was in high school, my math teacher would play Mozart during tests, claiming it would help us. It would drive me crazy, and I doubt that my grades improved as a result of the distraction.
For what it's worth, I have a BMus in Piano Performance. I love music... but never as "background noise"!
For me, it really depends on my mood and what I'm working on which type of music is appropriate. Some days it's rap and the flood of words doesn't get in the way even. Other days it has to be instrumental bluegrass or similar.