This is the shape that bears the closes resemblance to the camera frame, and the usual way of arranging a rectangle in the frame is with horizontal and verticals reference points.
		As mentioned earlier miss alignments with the frame are easy to spot.

	Rectangles have an obvious association with gravity, solidarity, precision and sharp limitation. They tend to be static and unyielding - formal.
	Compare the two images of the angels.

The perfect form of the rectangle - the square - exhibits these qualities most strongly. A major reason for these expressive qualities is that in nature they are rare and most of our reference to it are with man made constructions like buildings etc.

	For a rectangle to appear rectangular in the image it must be photographed from square on, level and with a normal focal length or longer lens. This is why many photographer who need to execute this demanding work use a view camera where the perspective can be altered by changing the relationship of the film plane and the lens.
	Wide angle lenses and angled viewpoints distort rectangles towards triangular shapes.

This means the situations where a perfect rectangle is used in an image are usually formal and very well considered.

Perhaps copying a painting, or the facade of a building, a packaged product where perfect perspective is required. Check out Vanishing Point