A different, yet related approach to the one espoused earlier, is advocated
here.
It basically boils down to "shoot lots, work out what you like, get better, repeat".
The debate about whether to get it right "in-camera" or not will run and run. It feels to me like a false dichotomy. It never does any harm to get the picture "as right as possible" in camera, but really, can the viewer tell whether a photograph was taken level or levelled slightly in post? I think not, unless the film border is included in the image.
That seems like a generic test to me - "can the viewer tell?" Of course another question is "does the viewer care?" For horizons, they need to be either properly level or dramatically tilted - timid tilting just looks like the error it is. So for horizons, they need to be right but it doesn't matter whether they were right in camera or not - to me at least. One can examine other errors - composition, focus, exposure, colour - on the same basis.
I take Gary's comments as urging me not to settle for mediocrity when I could do better, and bravo - it's well said.