Thought I'd mention that I have, along with many people I have met along the way, experimented with processing fabric in various ways. Tie dying t-shirts with my son was always fun too, we just did them in the microwave and I learned some great techniques like making a plaid design.
Most of the things I've done I've followed directions and used fixatives and such, and I've always had very positive results both in my artistic endeavors and the staying power of the dyes. I think that is the feeling of most people I know who have had a hand-dye kick.
I've known a couple of knitters who were serious about dying their yarn, often spinning it as well.
Just want to point out again, that the one fabric in 40 years of sewing that caused problems for me was one that shouldn't have by price and brand and all that. It's like the slogan -- trust no one! Still, that has been a lot of fabric with no problems of all names and brands and quality.
And while not dye related, I've had brand name fabric that was fine unwashed but after the sizing and such was removed was pretty flimsy or otherwise hard to work with like remaining surprisingly stiff. You develop a good eye and a good touch, but sometimes things change by washing.
I was just thinking about this the other day, I appreciate fabric from a tactile sense as well as visual. And visually I might prefer one of a different treatment or finish than another. I miss that part of shopping in person, those of you who have one/some, cherish your LQS! Those of us without are missing out on a big part of the appeal.