For the most part I think it depends your chickens. The guy who used to keep chickens here, had some very generic chicken from mexico.. they did not like anything below freezing (32). They litterally would huddle so close they would suffocate each other, so he had to use heaters just to prevent that. Mine are EEs with muffs, and they seem to prefer the cold (though we haven't had any days below 10 to know how they do with single digits or below). They want out of their coop even on the single digit days, and have to be coaxed into their coop at night.
Orphingtons I know are known to do well in the cold hardy though, so really I wouldn't assume the type of chicken to be an issue.... Also, it is also a very dry cold here in colorado, where if I remember correctly Minnesota is a much more wet cold, which might have different results because of the humidity? IDK.
I personally would say attempting to keep any large area at or above 40 would likely be expensive, and kinda hard to maintain. Though, if you are noticing your chickens shiveirng or cold issues, it isn't a bad idea to give them a corner or small area they can get warm...