There is no question that, in this day and age, a book's cover is first and foremost a marketing tool. Font, artwork, and layout are all designed to identify the book's genre and to induce the bookstore browser to pick the book off the shelf, examine it and, ideally, make a purchase.
When I enter a book store, the first thing I do is glance at the covers of the books displayed on the tables of new releases and pounce on any that appear to be historical novels. What are the visual cues that make me reach? Of course, for the last several years, headless women in period dress scream historical; other clues include gilded or calligraphied fonts, reproductions of paintings, bands of landscape set between borders of rich color, still-lifes composed of period objects. Covers of historical novels seldom feature photographs; the color palette is somber and dignified; the artwork establishes or at least hints at the novel's era and the setting. Historical fiction covers have a weighty look quite distinct from cartoony chick-lit covers or the artsy, photographic covers of literary fiction. Judgment aside, one should know a book by its cover.
Although I must admit I've had my fill of headless women, I understand the purpose they serve. Readers of historical fiction know the conventions of historical fiction cover art and search out books that fulfill those criteria. An author needs to reach the audience most suited for her work; why confuse the reader or miss her altogether by straying too far from the visual cues she's looking for? If headless women sell books, then I say plunk one on my cover! It's the designer's job to get potential readers to pick up the book, the author's job to distinguish her book from every other headless woman book out there through the writing between the covers. Please just make sure the costume matches the time period and setting of the story as closely as possible! There is nothing worse than finding an English dress on the cover of a book set in France, or sleeves from the 1570's on a book set in 1530.
The entire concept of using the cover to sell the book is quite a modern one. Book covers in the sixteenth century served two principal purposes: to protect the pages and to demonstrate the owner's wealth and taste. Books were bound in a bindery, an establishment distinct from the printing shop. Often times patrons would buy the printed pages unbound and take them to the bindery, where they would chose the leather and tooling that appealed to them and pay to have the book bound that specific way. The leather panels were embossed or stamped with geometric designs, scrolls or floral patterns; the covers of nobles' books were often set with precious gems. The cover revealed more about the book's owner than the content or genre of the work, especially since the title never appeared on it. Imagine having to open the cover in order to discover what a book was about. Browsing in a bookshop takes on a whole new meaning! (For some beautiful examples of sixteenth-century bindings, visit http://www.cyclopaedia.org/16c/16cbindings.html .)