I have an irrational fear of burnt popcorn. A single black, acrid kernel ruins an entire bag of microwave popcorn for me; even after tossing out the scorched parts, I can't get the smell (or the memory) out of my head. So I usually stop the microwave at the very first sign of a popping slowdown... and end up with a bag half-full of kernels. It's occurred to me that I could get a lot more popcorn if I gave it a few seconds longer, but I'm just not willing to take the risk.
A few friends think I'm crazy. "Just let it pop!" I've been told. But no—I'd rather unpopped kernels than burnt ones. Of course, it'd be better to have neither. But popping isn't an exact science.
What about you? Do you err on the side of unpopped or burnt?