customize was my first mode of choice. i would do main title and retain menu. this way i can put a dvd in and let the menu play while i go to the bathroom, backup another disc, check the forum, wait for others which are going to also watch the movie get seated, a whole host of reasons really. just insert the dvd and sit at the menu, instead of automatically playing.
but now i only use main movie to reduce compression
edit// as others have already mentioned. customize lets you select one title or any combination of available titles. then you have the option of menus and navigation.