A skim coat is a thin layer of plaster or compound taken across a whole wall rather than just over the joints. It costs more than taping alone, and on plenty of walls it is money that will never show. On a few, it is the only thing that gets the result somebody had in their head.
The deciding factor is nearly always raking light — light that travels along a surface rather than hitting it square on. Hit a wall face-on and small variations vanish. Run the same light along it at a shallow angle and every one of them casts a tiny shadow. That is why a wall can look perfect at midday and show every joint at four in the afternoon, and why nobody notices a problem until the furniture is in and the lamps are on.
So the walls worth skimming are the ones the light runs down. A hallway lit from one end. A stairwell with a window at the top. A long wall with a window at one end and nothing at the other. Any room with wall-washing downlights close to the surface, which is a lighting fashion that is very good at exposing finishing work. Large open rooms where one wall catches low winter sun.
There is a second case that has nothing to do with new work: old walls that have been patched repeatedly over the years. Every patch is a slightly different material at a slightly different height, and past a certain density of them, flattening the whole surface is quicker and better than chasing one more repair. The same applies to a textured wall someone wants flat — the texture comes off, and what is underneath will need skimming to be worth painting.
And the walls that usually do not need it: bedrooms lit by a central fixture, anything getting a flat matte paint, anything about to be covered, and rooms where the light arrives through a window in the middle of the wall rather than at the end of it. A properly taped and sanded wall is genuinely good enough for those, and paying to skim them is paying for something nobody will ever see.
The honest way to decide is to stand in the room in the evening, with the lighting that will actually be used, and look along each wall rather than at it. What you see then is what you will be living with.