I find that I nearly always work better without music on, or at least I do it faster. It's just a simple fact that on the days I listen to music at work, I probably only get done... maybe eight drawings, maximum. On days I don't have music playing, I nearly always have to resize my picture so I can draw more. Just counting today, I had fourteen finished drawings. That's almost twice as many drawings as a nonmusic day.
Only problem is, I like working to music. At any rate, it's a habit.