I did this as well (with "Fuel" by Metallica) and it is awesome. I don't have a super computer, so it lags my game the very first time I build a dispenser (or when I near someone else's dispenser) after loading TF2. As they said on FPSB, it takes place of this low pitch humming that the dispenser gives out, and that is, in all practicality, inaudible. I would highly recommend it, and it only takes a couple of minutes.
As Woodstock said, choosing the song is the most difficult part.