Are you making the petals in advance so that they can dry well? If so, well they will be fine for 24 hours or so ahead of time, possibly longer. The ribbon, is it made of fondant or material? A lot depends on the fabric. For instance, satin will show up grease spots and like you were advised, contact plastic or tape or something is good to back it with - even ironing on waxed paper so the wax creates a barrier. To do this you need to put a towel under the ribbon, place the ribbon on top of waxed paper, put another towel in between this and your iron.
Otherwise some of the wired ribbons made from organza and such are not affected by grease. Personally, if I was doing this, I would do a test with some of the icing and leaving some petals on it for a day or two and also a small piece of the ribbon on some icing for a day or two to see what happens. That way you won't get any nasty surprises.
Hugs Squirrelly Cakes