A few of my friends who are published also do maps. For one, the inspiration for the entire book/series stems from her maps. Another doodles something on scrap paper to solidify his ideas. Another keeps it in all her head. Another plots it out on some confusing computer program I can't remember.
Of the four authors (small sampling, but all I have), none of them submitted the maps with their books. My friends who can actually draw maps often mention to their editor (once the book is sold) that the map exists and the editor will generally look at it for consideration. Of these maps, the author's map is what got printed if a map got printed. On occasion, the editor will ask if there's a map and, if not, the author has generally produced one, either themselves or get an artist to do it, and that was the one that got printed. No professional artist hired by the publisher has ever been involved with my (limited) examples.
But maps, like any other artwork, doesn't get discussed until the book has been bought and turned in in its final form.
Hope that helps.
