I would lay some tiles out on it first in order to see if it looks retarded or not first of all
It depends on how you finish against the reveal as well. If you don't tile the cill, or the reveal, you need to finish against the cill in the same way you do against the reveal. Otherwise it'll look weird.
You could leave a small margin, like two centimetres or something, on vertical surface below the cill, or you could skip tiling that surface altogether and just abrade it, skim it, and paint it.
Mixing painted surfaces with tiled surfaces is all the rage among designer people over here. Regular surfaces are tiled plainly, with some small decoration perhaps, and then there's often a feature wall with mosaics or something. Irregular surfaces, especially in older buildings, are often painted instead of being made regular and flat. The color of the paint is usually the same as the main tiles, so the surfaces don't contrast so much.