I've seen cog belts that have the tooth side of the belt riding on the smooth face of an idler pulley. I'm doing some mock ups now for my supercharger belt routing but am using an 8 rib serpentine belt and not a cog belt. Is there any harm in running a serpentine belt's tooth side over the idler pulley if its a smooth face pulley and not a groooved pulley (like the alternator pulley?). I can route the back of the belt or the smooth side of the belt over the smooth face pulley but that doens't give me the best tension for when I install the autotensioner.

 

As long as there are ribs on the sides preventing it from walking off the pulley you shouldnt have a problem