lI tried to find images (which I know I've seen) of people sitting at their sewing machines. It's a very good question, because any chair that has a seat that slants upward in the front is uncomfortable to use for treadling. Here's a page on ISMACS that has pictures of people sitting in chairs:
http://www.ismacs.net/wheelerandwils...ne_factory.htm
I use an old chair that has a little padding, straight back, and medium height legs.
Linda