I live in Washington State, where every server is paid at least the state minimum wage before tips
. Because of this, I am unconcerned about how much I tip and have no hesitations on the few occasions where no tip is warranted. This is not the case when I travel to Michigan, where a server is often earning less than half of that state's minimum wage and needs to make up the rest via tipping. In those cases I tip imo generously.
Here is a handy guide that shows each US state's minimum wage vs. the minimum wage paid to servers: