I recently made a cake with ribbons around each tier. (In my photos-Black & White cake) My cake was covered in MMF and then I used actual ribbons and attached them to the cake with royal icing. I have seen fondant ribbons used, but I wasn't brave enough for that! Hope that helps!
I picked up a trick from a very experienced baker in my area. she actually puts crisco on her ribbon to keep it from gettting greased and discoloring the cake. It also adheres the ribbon to the cake too!
I decorated my little sisters wedding cake with ribbon. It was BC and she wanted a sheer purple ribbon. I tried BC to stick it to the cake but you could see the BC, so I came up with my own plan. I took regular silver straight pins and stuck them through pearl beads, with a dab of hot glue between pin head and bead, folded the end of ribbon on one side and stuck the pins through the ribbon into the cake. I used three pins for each tier (3), it worked and she loved it. 
I think that would probably be easier, then you don't have to fight with it to try to get the bow in the middle of your ribbon while you are trying to put around the cake. I would do the ribbon around the bottom 1st, make your bow and attach your bow w/your pearl pin. The pearl in the middle would look great, or you could hide it in the bow. I think you could also use other kinds of beads or crystals (with holes).
I used real ribbon on the one wedding cake I've done.
I didn't grease my ribbons, they were a synthetic fiber that wasn't absorbent....but I was prepared to if it looked like they would spot from the BC.
I picked up pearl-headed corsage pins from Hobby Lobby to attach them.
HTH
Tim
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