While I prefer to paint rooms completely, including the ceiling, I sometimes need to paint a single wall, or even a limited area of one wall.
For example, the kids may have drawn directly on the wall, or in the entrance area just one area of the wall is very smudged etc., and there may be a lot of furniture on the "good" walls which would just be not worthwhile to move around.
That said, what are good techniques to avoid ugly edges where your new paint meets the old? For example, at wall edges, would you rather stick on your masking tape on the wall that you are painting, or on the adjacent wall? Or would you avoid hard edges where new paint meets old, but somehow smooth the paint into the old areas? I saw an old professional painter do that once and it turned out fine, but I really have no idea how I would do something like that consistently.
I am mostly talking about the commonplace western europe white interior paints, not latex based paints.