My tipping policy is, I tip on luxuries and I expect to pay for expertise on necessities.
So I tip waiters, for fancy-schmacy coffee, for manicures, hotel staff, tour guides, cab drivers, spa massages, etc.
I do not tip my doctor, my mechanic, the grocer, for regular brewed coffee, etc.
My thinking is, I don't need any of the first group, they are simply things I choose to pay for in life to make my life nicer. I don't need to eat out, or get a fancy foamy flavored coffee, or go on vacation, or travel in cab vs a subway, etc. these are the little life perks I treat myself to when I can afford them. And to me part of being able to afford a luxury is tipping for the service. Tips to me are like whipped cream - a special extra something you put on top of an already indulgent thing.
The second group, I do not tip. I expect these people to be trained professionals who do a no nonsense job for an agreed upon professional fee. I'm not mentally in happy-land getting these services, and I certainly aren't thinking of these things as life "extras" so I'm not in the mindset to pay extra in the form of a tip.