When moving through a new group of pictures, I typically do a rejection pass first, then go back and rate the rest. If XnView had a flag control on the thumb, in my work-flow I would go through the thumbs and click a black flag for each rejected image.
Here is how Lightroom does it. Notice how Lightroom dims the black flag thumb image (i.e., rejected) and puts a white border around the white flagged thumb (accepted).
