I blanket when it's extremely cold (20F or less), when it's windy and cold with wind chills into the 20's, or wet and cold. If my horse is one who grows a decent winter coat, I just use a midweight blanket, on the ones who do not grow winter coats, I use the heavyweights. After many years of experimenting with different cuts of blankets, I've come to the conclusion that this one is my favorite:
StormShield® CONTOUR COLLAR Bellyband Turnouts in StormShield® Contour Collar Euro Turnout Blankets at Schneider Saddlery
This style is really durable, has a 2 year guarantee, Schneider's stands behind their products, and it fits everyone from the QH's to the Saddlebreds to the Arabs without trouble. I haven't had wither or shoulder rubs, it doesn't twist, and I have some that are 5 years old and still very much in service with few repairs.