"Of course by the level": said the hostess when asked how to hang the curtain
I always find out from the client his preferences, so as not to redo it later, standard questions and the same answers. I often joke: "The owner said the chandelier is on the floor, so it is on the floor."
Installing curtains
I don't like hanging curtains, manufacturers are trying to be creative, but in fact the quality is getting worse from time to time, or I'm just “lucky” and the owners are trying to take a bright, but cheap product.
Evaluate the components, the manufacturer did not even remove the jumpers from the hooks, packing them, as they left the conveyor, they had to be cut.
So by level or not
Everyone knows that the curtain is located horizontally, but a pipe (heating or gas) and, of course, the ceiling can go nearby. Here they can be "overwhelmed", the heating pipe in any way will be with an angle, since the circulation requires a slope.
And if you arrange the curtain on the level, then visually it will look obliquely with a curved ceiling, or with such a pipe. Therefore, I expose according to the level and call of the owner, here is the horizon, as asked and ...
I have to raise or lower one edge so that it is visually even, but what for me, a person to live here, is up to him. As a result, out of a couple of dozen hung curtains, I remember only a couple of them set according to the level.
More history
I had a case when had to shift a straight apron from tiles to the level of the curved ceiling, at the level it looked askew. Such is the story.