I'm new to cake decorating and was just told that you can't put buttercream decorations on fondant because the oil from the buttercream will leak onto the fondant giving it grease stains and to use Royal Icing decorations instead. Is this true? I've seen buttercream used under fondant many times to attach the fondant to a cake, so it doesn't make sense to me to not use it over fondant.

I have used buttercream on fondant before. I have stenciled buttercream on fondant before with no problems. Sorry I couldn't be more help.

I have also used buttercream on fondant with no problems.