I'd like to add my two cents to this old topic, in case there's someone like me reading dead discussions.

Maps are always made for someone. They should contain the labels that are important for the (real or in-character) map-maker and the (real or in-character) intended audience. To take the LOTR example, Ithilien is labeled along with Gondor and Rohan because it's very important to the plot, and the function of that map is to illustrate the book's story.