I find that doing sketch thumbnails allows me to plan my images much faster, and create more realistic and less stiff poses, its much easier to see how positive and negitive space works together with a thumbnail. plus I can do it anywhere, sketch out the concept, so I don't forget it later, its a lot harder to do that directly in poser unless you want to carry a laptop around with you and fire it up every time you have an idea. I dont often do detailed sketches, more often they are simply blocked in shapes which give the overall gist of the idea.