For basic service, as in the person did their job and covered all of the basics of their job then they get 20% regardless of the cost of the service.
If I think the person went above and beyond, then I add on from the 20%.
It takes a lot for M. to decide that someone doesn't deserve that basic 20% minimum. It has to reach the point that their service sucks, they're rude, and any mistakes are their fault and incompetence and not the fault of kitchen staff of anyone else, and I have to say something to the manager to complain (and in doing so clarify that the situation is in fact the server's fault) that's when I revoke a tip. But if I have to do that then I'd rather request a new server who does deserve the tip.
Of course, if they're in training, that's different. I have more patience in that instance.
If I have to wait for my service, such as waiting for my hair stylist, I don't detract from the tip. That's just... it makes no sense. It's idiotic. You tip based on the products you receive aka the cut plus dry, make-up job, color, highlights, color rinse, perm, whatever it is you're having done to your hair, eyebrows, lip wax, etc. You tip on THOSE things, not your wait time. If you don't get those things every time then your tip amount will change.
Of course you have to wait for them to finish up sometimes. That's life. If you can't wait then go to the appointment desk and make a new appointment or find a new salon. You don't dock the tip because you had to wait.
EDIT: Do people really not understand the reason for tips? That tipping is NOT icing on the cake of a paycheck in businesses that have tipping? Tipping is the MAJORITY of a person's paycheck in these businesses and when someone is a server or even a hair stylist especially in a place like Cost Cutters, the only money they make is that tip. They don't make commission. And these are the only jobs where people seem to think it's all right to decide how arbitrarily how much their service is worth and therefore how much of a tip they're going to leave.
That's why a FLAT RATE TIP is 20%, and if you can't cover a minimum of 20% then you can't afford to go out and even get the service done. Can't cover all of your services you're getting at the salon AND the appropriate tip? Then wait until you CAN cover it. Can't cover your meal AND the tip? Then don't go.