for me, it's just like the appraisal and annual salary review in a lot of companies...

performance within expectation /job spec i.e. satisfied service =no tip

exceed expectation i.e. very good service = tips

'below expectation' i.e. lousy service = no tip (you won't get a salary decrease for below expectation in your appraisal

the only circumstance is that you know the owner or a frequent customer of the restaurant. I feel like obliging to give tips but still i pay 10% only.