I have full overlay cabinets without a visible light rail, and small flush molding as a spacer for the ceiling. My sink window is a four foot wide, counter level garden window (i.e., pushed out for plants) with ceiling high cabinets up to the molding on either side. It would have looked kind of dumb with all this going on and nothing over it. Rather than a valance, I have a bridge cabinet. It is a foot high, flush to the same molding at ceiling height, and connects the two sides. There are a couple of knobs on the front, and a decorative heat register. Inside are a couple of lighting cans directed through the bottom at the sink, and space for spare lamps (or whatever one might want to hide).
Valances aren't done as much now mostly because a lot of people are putting in standard (rather than custom) sized cabinets, and either not taking them to the ceiling or soffit, or filling the gap with furniture moldings. That makes it hard to design in a valance that is appropriate and integral to the cabinetry design.
Cali Wendy's sink, in the linked thread, shows how you can make a valance look fresh in a classically styled kitchen. Notice the shape, like the white inset cabinets, references the past, but is also crisp and modern. Insetting it a few inches highlights and differentiates the window area more than flush would.
What is very out, until the '50's/'60's suburban kitchen becomes the next trend, is the scalloped or challet style valance. I also wouldn't do a straight board unless it were for a really rustic look. Something designed for the kitchen that's in proper proportion and style can be beautifyl.
Q