10% for lousy? Seriously? They're only required to report 7.5% on tips for income tax purposes. I wouldn't give more than 7.5% for lousy, I would give less. In a lot of places, the tip money is pooled for all the employees so the lousy ones don't get punished versus the good employees. If I enjoy overall service, I tip great. If not, but the waiter/waitress is good I slip money into their palm.
I do the same thing. I pay with CC and give cash to good waiters. I also cash tip to kids working at fast food and subshops.