When you attach a ribbon to the cake is there just going to be a back side because of where you join the ribbon? Is there some trick to tucking the ribbon or keeping it from fraying?
You could put a bow where the ribbon meets. Or tuck under the free edge and pipe some "buttons".
Agree with Rose....or make it the "front" and hide seam with a bow. If you have good sharp scizzors, you shouldn't have any frayed edges. If you don't want either option, make it the back and add a smidge of buttercream to "seal"
If you have ribbon that won't stop fraying, you can use fraycheck on the edge. It depends on the ribbon, with a straight cut, woven ribbon will fray, no matter how sharp your scissors are.
I would just add a bow tho.
Or press back the edge with an iron for a clean edge and just have a straight clean seam.