I have the impression that a restaurant server's 'taxable tips' are estimated by the government. Not sure why I think this.

I tipped my bulldozer guy on a multi-thousand dollar job at my ranch. Last year he arrived with a dozer, backhoe, grader(!), water trailer, and several skilled helpers. All this was far above his usual annual visit with a D-9. The results were fantastic and I appreciated the efforts.

Meanwhile, the topic at hand can be augmented with info from wikipedia, which I love dearly. So, here's a link...
http://en.wikipedia.org/wiki/Tip_%28gratuity%29