Has little to do with duty hours on any given day. I worry about fatigue when I look at a trip and see where delays and other factors might affect my cognitive performance. Fatigue is insidious and is not easily recognizable in the early stages, which is where you might start making mistakes.
I believe all pilots push it, and it almost always works out for us because we often work in a safe environment. But if I know I'm tired, and I'm flying to someplace with an increased risk (weather, terrain, approaches) that's when I seriously consider calling in.