I guess I'll be the "jerk" here. If someone is working for themselves (setting their own prices, doing it from their own home), I expect them to charge what they want for the service. They are a business owner. I don't tip the owner.
When I go to a salon (getting hair done by an employee or someone who has to rent their space in the salon), then I tip accordingly. These people tend not to be free to set their own rates, and aren't paid as highly. For highlights, it usually takes a couple hours, and I generally tip about $20-$25 (the service is usually around $100-$125).