I think your observations are spot on. What comes out of the typical gas station pump is already blended for the climate at hand and requires no thought or special action on your part.
However, the fuel that gets pumped into a car is seldom there for more than a week or so, and there's no opportunity for the climate to change between the time you pump and the time the fuel is consumed. Your tractor, on the other hand, can be filled with fuel in the summer or early fall and still have that same fuel in it when January rolls around. This is the situation you expressed and is exactly where you get in trouble. I got burned (or gelled in this case) by just this a few years ago.
Unfortunately, there's no way to know when the fuel distributors start treating fuel for the cold, and when that treated fuel is in the tank you are pumping from.
Given this, I'd treat fuel at this time of year if it's not going to get consumed within a few weeks to a month. Treatment is lots easier than un gelling a fuel system. If you run through your fuel fast enough, this is a big don't care.
For those of us with larger storage tanks, this problem shows up more often since the likelyhood of buying fuel in the warm months and still having it around in the cold goes way up. Come to think of it, I've got about 20 gal in my storage tank that I better treat before it gets too cold.