Thank you all so much for your input. It sounds like most of you -like me- do tip the salon owner between 15-20%. I'll continue to do so and try not to feel weird/doubtful about it.
I hate the whole tipping issue, too. In restaurants it's fine since it's so clear cut; I tip 15% for average service and more for great service. Everywhere else, e.g. in cabs, I struggle to figure out the right amount.
And then when I'm in Germany, I'm so used to the American system that I tend to over-tip in restaurants and everywhere else. People do tip in Germany, but very moderately, e.g. you'd round up a tab of 9.50 to 10 Euros at a restaurant. There isn't a clear formula like the 15-20% in the US. I can never figure out what's enough vs. over the top and always ask my friends or family members to do it for me... Stressful indeed.