We have no rules, my horses are very different, and my BO is a wonderful wonderful person, and if they need to be changed its preferred that I text what I'd like them to wear.
I have an older horse who runs quite cold, it was in the mid 60s here today but very windy and chilly, he had his back on track mesh sheet, with a rain sheet and was very happy. He hates being cold, and will have layers upon layers on in the winter, way more than anyone else in the barn. He has a trace clip.
I have another horse who is in the middle of the blanketing spectrum, he was out today without a sheet and was happy to be muddy and dirty.
It's so dependent on the individual horse, I don't like them sweating under their blankets though.
Depends on my individual horse and it it's wet/windy.
If I was putting heavyweights on everytime it was 30, I'm sure my horses would be living in them for half the year (Wisconsin).
It was around 40 I believe when I left the barn and a little colder most of the day. Filly was in a light sheet, mare was naked. Filly is a wimp and hides in the shed without clothes. Mare really doesn't care until she's wet or if it's too windy.