In the USA, your minimum wages are so abysmal that tips are borderline necessary for the servers to have any decent standard of living. So long as my service wasn't actively terrible, I'd tip. Food is so much cheaper in the US than in Australia, that tipping a server will bring the price up to what I usually would pay for in Australia (our currencies are roughly worth the same these days). So I would tip, even if the service was average. In fact, I PREFER average service. I prefer it if the server just gives me the food (not rudely), but just hands it over and leaves without a word. I don't like it when the server tries to act all "buddy-buddy" with me - I sympathize with the fact that they have to act nice for the customer, but it always bothered me since it's so obviously fake.
But yeah, if I lived in the US, I would tip people so long as they weren't actively rude or really bad at their job.