Indeed, if you didn't build a floating floor, you could stop the whole place from shaking due to the mass of the concrete. However, you may want this effect, in which case, go ahead. I agree with gregz, it would make the whole wiring thing really clean and professional. Still, going with good thick carpet and pad would yield great results and prevent you from having resonance issues with the floor. In fact, a friend's amateur studio is built in his renovated garage and it's just carpet w/pad on the floor. He just has lots of sound absorbing stuff around the place and an absorbing ceiling. It's nice and dead.
In the end, it's all about the sound and look you want to create (and your budget).