Actually thinking back, it might be more interesting to fuck someone who was still living. Unless their memory is zapped afterwards or something... That way, if its some living person, there would be potential for more... not necessarily more sex, but press attention, fame, extortion, book deals...
Yet if you went back in time and screwed Cleopatra or something, who would believe you? If its just going to be a one time fuck it probably won't be very meaninful, so might as well get the most out of it by thinking about the future.