I quit doing them because my elbow locked up and I had to keep it in a sling for 2 weeks from the strain of scratching in such tight strokes. I never did one again since that incident..but you can't live in fear and being an artist, I have to expect a little pain for pleasure.
They're fun to do, but time consuming. I think I might take them up again, but we'll see. I need to find my retractable X-acto knife..it was my favorite and I feel more comfortable with it =)