In a formal setting, we would typically specify a pelmet.
In a more contemporary interior, I prefer a straight box pelmet covered in fabric for a cleaner look.
In a classic scheme, I would shape the pelmet to soften its appearance. For added interest, we often incorporate contrast piping along the base or a decorative trim such as tassel or gimp, which ties in with the curtains and the room scheme.
In certain situations, particularly bedrooms, a gathered pelmet can look wonderfully soft and romantic.
We have also recently installed a pleated pelmet in the morning room of a Georgian country house, which worked beautifully.