I'll be making a 3 tier (12, 9, 6) wedding cake soon and have to put blue satin ribbon around the base of each cake iced in white SMBC. Do I put the ribbon around each cake before or after they are stacked?  I'd like to put it on before I stack them but not sure if that makes it harder to stack. Should I coat the ribbon with shortening before I put it on?  Or put a backing on the ribbon instead?

Personally I would put the ribbon on after stacking the cakes to make sure it sits flush with the tier underneath (and to hide any imperfections). I'd use some piping gel or royal icing to stick it on, but I generally don't put non-edible decorations on the cakes I've made so far, so I'd be interested to see what the most experienced bakers on here have to say :)

 I put my ribbon on after the cake is stacked, I've just always done it that way. I also just use a small dab of chocolate to secure it at the back. I've tried coating the ribbon with shortening before, and it just made the ribbon greasy. 

I add ribbon after my cakes are stacked. If there's a possibility of icing etc seeping/discoloring the ribbon then I measure out wax paper the same sz and length and spray glue or glue the ribbon and wax paper together with a glue gun and then apply that to the cake.