This might be a dumb question but here is goes!

What's the best way to hold  a ribbon on fondant and buttercream cakes?  

I use white Royal Icing. Works great.

I don't use buttercream very often but when I do it's Italian Meringue Buttercream and the ribbon sticks to it without any assistance.  BUT the butter in the buttercream soaks into the ribbon making it look blotchy,   SO, purchase your ribbon one shade lighter than needed then run the ribbon through liquid corn oil and ripe most of it off.  Now the ribbon is one shade darker, the shade you actually want and the oil makes it stick to the buttercream.  With Fondant, just buy the ribbon one shade lighter and run it under the faucet then thread it through two fingers to get most of the water off and apply to the fondant.

AHHH thanks!   lol   the ribbon I have to do in a couple days is very silky and I didn't know what to hold it with. I was about to use scotch tape!! lol jk,