I'm getting ready to do a wedding cake and we're doing super tall tiers, so we're adding 2" styrofoam pieces each cake tier. I'm putting a cardboard round between the cake & the styrofoam but would you cover the styrofoam with royal icing beforehand so that there's no chance of little styrofoam pieces sneaking thier way into the buttercream? It's not the styro that dummies are made of, it's the normal craft kind. (not the green florist stuff though)

Yes, I would definitely give them a "crumb coat". Once I've even shrink-wrapped the styro: makes clean-up a breeze!

You do mean a crumb coat with royal icing, correct? Royal icing will stick to shrink wrap......... wow, I'll have to try that sometime.