Before you print to CD's, change from Wireframe to Hidden Line.
In Wireframe, I'm seeing a centerline for both rectangular and round. Why Revit displays a centerline for rectangular, I haven't a clue, although the duct tag does start there.
Trust me, this Yank never put centerlines on rectangular ducts before (actually, didn't put them on round either, but our previous software didn't have that option).
Ed