In some places you would be taking food out of her mouth.
Where I live in Sth America the tip included in the bill goes into a kitty, which is then split to form the major portion of the wages of the staff. Not just your waiter, but all the staff on deck. If you get bad service or are a tight cough you can request to have the tip excluded from the bill, which is accepted without question. This does mean however that you are penalizing all staff equally, even if it is just the **** who served you that was the problem child. However, part of the kitty is taken by the establishment, so if you really liked a particular waiter you can either give them an addittional separate tip which they keep all of, or you can request the standard % tip be left off the bill, and instead give the tip again direct to the staff member, so they get all of it.
I agree that auto % tips included in a bill may remove to a degree your power to reward (or not) according to the level of service, but just be aware that in many places this may be the sole or predominant source of the staff income.