In no other circumstances is a unilateral and after the fact negotiation acceptable. Would YOU work for a variable salary with no clear criteria on what affect it, no guarantee of salary regardless of your skills an effort and for which studies have shown that the colour of your skin, your sex and the mood and social background of the customer has more effect on your salary than anything else?
If the service and the food is good, I tip 15% and go back for more and talk about it to everybody I know. If the service or the food is bad, I tip 15%, never go back, and let everybody I know, know about it.