Update: Menu items now have the class “menu-item-has-children” that can be used for styling thank to this core ticket. So, the menu walker is no longer needed.
I think it’s important to give users a visual cue when menu items have a drop down menu.
One way I’ve handled this in the past is to use the Superfish jQuery plugin, which adds a class to any list items with children. This allows them to be styled differently- with a background image of a down arrow (for instance).