Your hips, knees and elbows should be at 90degrees when you're at a computer, so I guess I'd carry that over to a sewing machine too. Your ears should be in a line with your shoulders which should be in a line with your hips. Either or both in a forward position will strain your neck and upper back muscles.
I'm a PT, I teach this to patients every day. But do I follow it? hehe. I do have a BackJoy seat in my chair now that helps too, and we won a 24/7 chair at a fundraiser auction, so that should help too!