The side you are mounting to the body gets the paint so the top side that gets looked at is shiney. It is all up to how you wan the finish to look.
I have done both and both look fine.
Peter
Ok I am totally assuming that you have a body that is covered in some sort of plastic. If so you would want to paint the underside of the wing. If yo are talking about aftermarket wings than well I think the top would be cool.