I would like to wrap a fondant ribbon with bow around each base of three tiers separated by pillars. I would like to do so that no cake board/base is exposed, only ribbon. Should I be concerened that the fondant ribbon will slide off of the cake once the cake sits through the reception and the IMBC becomes room temp? If so, I will have to have the cake board show a little so the the fondant cannot slide off. Anyone with experience of using this combination of fondant on IMBC? Thank you much!!

I've never had fondant accents ever slide off of room temp IMBC. As a matter of fact, almost all of my cakes are IMBC with fondant accents. Feel free to look at my gallery.
As long as they aren't big thick pieces, you wont have any problems.
HTH,
Sharon