I blanket my horse after they get a nice thick winter coat and only if it's wet out, like rain ir snow. Also if the horse has health issues or thier age permits it. I have a 19yr old mini who tends to lose some wieght in the winter very fast so I blanket her on the days it gets very cold or it's a wet snow that's heavy. But in general, I try to prolong putting a blanket on my horses, I prefer to increase thier fiber intake.
I actually prefer not to blanket unless it is absolutely necessary (determined each year on a horse-by-horse basis - ie if they are sick, changed climate too late to hair-up, etc). 99.9% of the time, I don't blanket at all during the winter -- here in Indiana or back "home" in the Idaho mountains. Our horses have always managed to weather just fine with their natural coats.