never - seriously. When I started a bought a can of spray starch and 4 years later, it's still sitting on the shelf.
I prewash everything, I hate fabric that feels like paper, I love the flexibility and being able to "work" with that.
I come from a background of sewing clothing and learned how to deal with fabric that was difficult, so sewing on quilting cotton is easy. My feelings are to learn to work with fabric instead of trying to turn it into paper.