Sirius, vertical is the key to a 'true' reading of a photograph.
Horizon lines, when no attempt is made to represent reality to the viewer, are a false direction - impression given to the viewer who in general wants to read the image as it exist in the real world.
Deliberately slanting a camera for fun, to capture an entire view, as from inside a cave, looking out or to make some kind of statement about what is shown.
Well know examples of this include a wildly shifted view from the beneath another person, to indicate that the person (photographer) is on the horizontal plane to say, with a disapproving girlfriend learning down or to a building, etc.
Party pictures, such as those used to indicate the fun using, i.e., an Instax mini camera/film, on advertising and packaging is another way to use a deliberately shifted horizontal position, but it's an exception that is accepted as part of the 'Fun' of the product in use, in marketing.
Personally, other than polaroids of friends, I've never felt a need to use a shifted position.
Vertical, in my opinion IS the correct way to set up a shot, horizontal is always a side that will take care of itself, once a true standing position is established.
Cheers.
So you never have a horizon line off kilter?
