I wish I had a mansard roofed house--but I only have a 1908 'four-square'--I hate that term! Anyway, my attic is finished in original lath and plaster, and it has curved joins at the top of the knee-walls, and also where the angled portion joins the flat ceiling. It looks very nice but does give problems in painting--I have painted from the baseboard to the very top all the same color, rather than deciding where to stop the wall color. :)
At a guess, and remembering episodes of TOH, I would say curved joins were the norm.

It depends on the skill of the installer. You might not have a choice.

Large radius (inner or outer) joints are very simple for a plasterer, not so easy for a drywall person.

The curves help disguise the boundaries between the various planes, especially if the paint color is the same on both planes.

I have always carried the knee wall color up to the ceiling curve, than changed color their.

If there are dormers or other square ceiling-wall joints, lining up the color change line with them seems to look best.