If you want to see the correct aspect ratio as shown in the theater you leave it in original. You will see black bars since most movies have different aspect ratios.
If you want to fill the screen set it to fill. You will either have to stretch the picture or cut off parts of the picture to fill the screen,
Since you have an 80" TV, for myself I would opt for original but, whatever makes you happy is right. On very widescreen movies you would lose quite a bit or distort the picture.