I’m not quite sure what you mean. The wp_nav_menu() will automatically use the Walker_Nav_Menu Class by default. It’s not necessary to use a custom walker for a drop down menu ( or submenu ). The wp_nav_menu() accepts a few arguments and one of those is depth. If the depth argument is set to 1 then it will limit the menu to top level items.
Really, it’s up to your theme to use CSS and style these dropdown menus so that they are functional.