For me it is the distant mountains that stand out in 3,4 and 6. 2 has some but isn't as dominant. You could draw the line at the base of the mountains. Of course the more tilted the more the picture will have to be cropped which may change what you had originally intended.
I have to put a conscious effort into making sure I'm level with the horizon when shooting landscapes. Otherwise I'm straightening with NX2 afterward. I usually shoot a little wide then crop anyways.
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hey sorry for the late reply, i understand the problem with handing conflicting planes in an image, as a rule of thumb the horizon should ALWAYS be level (unless ur doing something dynamic by tilting it, theres always a way to break a rule).
the other plane my be slanted, but thats less of a problem than the horizon being slanted. sometimes it helps to be perpendicular to the scene, i dont know the actual physics/visual proporties of it well enough to explain, but sometimes when im taking a picture of a scene from an angle, different planes appear skewed. i dunno if that makes sense but i hope you get what im trying to say
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Disclaimer:
Any views or ideas represented in this post are strictly my own. I reserve the right to
be wrong.