Now that I body clipped my mare (because she sweats alot when she's worked, and it takes forever to cool and dry her off), so she is blanketed at night, and will have a sheet on during the day, when I get a new one; when it's really 'warm' she won't have a blanket on at all during the day.
The two other horses I take care of, aren't blanketed at all, and I never see them have any issues; they have a barn they have free access too, and get plenty of food. They are a pain to dry off, but they aren't being ridden, so I won't body clip them at all.
This is a first ever, I've blanketed (or body clipped) a horse during the winter, as I grew up in MN and never had a need too; they didn't sweat in the winter during their workouts...
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."