20% for all.
Unless the person is the owner of the business, than it is acceptable to tip less....I would say minimum 10-15%.
EDIT: I suppose the second answer is right, depending on what country you are from....I am answering from the U.S. But I have to chime in on the third answer and say absolutely NOT.....in most places a tip is NOT optional.
Most of the time, when one is in an industry where it is customary to leave a tip, they are paid LESS than minimum wage....because it is assumed they rely on tips for the majority of their income.
Of course you should get outstanding service everywhere you go....but lets be realistic....did the last cashier at the grocery store make you feel 'exceptional' or did she do her job? Was the last cop that pulled you over 'outstanding' or was he just doing his job? In the United States, the people who we tip rely on it as the majority of their income, so it is not acceptable to stiff them if they do a decent job....If you would like to be my guest, but I suspect you will be ingesting more than your share of undesirable things if you frequent the same places that you regularly stiff.
If you disagree with me, I suggest you write the restaurant and bars you frequent, requesting that the waitresses and bartenders salaries be raised to compensate for gratuity. If they paid their people a decent wage instead of relying on tips, you would pay triple for your next sandwich or cocktail.
Good luck to you!