M.W.
You are already saving twenty dollars, so if you don't tip her the same, I think it would offend her. The ten dollar tip was for her "good work" and now anything less would seem to be saying "you are worth less now". She will still be doing the same good quality work for you.
I always cut my own hair (all one length), so I am definitely not an expert in this field. I am only looking at it from a relationship viewpoint.