Thank you srmoment for posting this and to all for contributing to this interesting thread.
Just to add my 2 cents, I feel that making a photo "look balanced and good" is more important than getting things absolutely "level". Of course it's subjective, but sometimes a crop may look more interesting when its not perfectly level. Lens distortion, the angle of view and perspective can all distort reality. Due to the angle of view, sometimes, getting one line straight, will throw another line off. Cropping by what looks more pleasing may be the way to go.