Just wanted to add that when you put a blanket on a horse, it lays all the hair flat. The warmth in the horses hair is kept by the hair being fluffed up because the hair being fluffed creates air pockets that hold in warm air. If you are going to keep them blanketed all the time, be sure to have one that is plently warm enough because otherwise the blanket is pointless...
Now as far as the blankets go, I would get a heavier blanket at least 1200 denier with ripstop. Weatherbeeta has these and they are great. This way if they do get a snag, it will not rip the entire blanket and they can be repaired, I usually have about 50 dollars worth of repairs between 6 blankets at the end of the season.
If the barn in not drafty and damp then taking the blankets off should be fine if the BO will do it, unless it is super cold, like going to singles or negative digits, then I would leave them on.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"