Gerald,
I almost never use negative rake scrapers. They are a clean up tool, and for most of what I do, they are not needed. The main use for them that I have found is for end grain on boxes. They do work better on harder woods, and not as well on softer ones, and for sure they work better on end grain. If you get a chance to watch Stuart Batty use them, he is the best I have seen with them. He likes them for 'interrupted edges', and getting in close where you don't want to have to sand other than minimal. For the bowls I do, a shear scrape, or a high angle shear cut works as well if not better.
robo hippy