I have heard both, even for horses that live in a barn let alone turned out.
From the trainers I worked for that braided manes it was important that the first part of the braid next to the crest was loose enough that when the horse stretch his neck downard that the hair would not pull out. And like you said it was redone every week.
Others I have worked for refused to braid manes but braided tails.(but unbraided when turned out for anymore than a day or two)
The only thing I could conclude was that the ones that had the very long manes were genetically blessed with one anyhow.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.