I do believe that Wanakee had it right - exposure to the elements, not to mention friction from one's clothes, really does make the ends of the hair more prone to split ends and breakage, especially when you have a fragile hair type to begin with. On the other hand, sometimes I think that I might get away with wearing my hair open as often as I like, without suffering any adverse effects, if I faithfully moisturize/oil/seal my ends while they are exposed.
Be that as it may, at the moment I don't take too many chances. I compromise by wearing my hair down about once or twice a week, mostly on weekends, and mostly at home after I've shampooed and roller set. I reason that on weekends I have the option of wearing tops that are cut lower in the back than my work clothes, allowing my hair to rest on my skin, not on fabric. I figure that my own skin won't subject my hair to any friction; plus, I like the sensation of my hair against my bare skin. /images/graemlins/smile.gif
I don't wear my hair open at work very often, but when I do, I wear clothes that are made of fabric that will be easy on my hair, i.e., won't occasion too much friction. Sometimes, if I really feel like it, I let my hair down after work when I'm at home, out of the wind and the weather.
And of course, I wear my hair down on those occasions when I want to look 'extra'. /images/graemlins/laugh.gif At such times I follow either my low cut back, or silky fabric rule (or both). Whenever I wear my hair down, however, I am extra careful about keeping the ends moisturized and oiled.
On the other hand, if my hair ever reaches my target length of the middle of my bra strap, I would probably wear it down as much as I want, (keeping the ends well moisturised/oiled/sealed, of course) since I would be focussing more on maintaining my length, rather than on attaining more.
Ok, I know this post was long and rambling. In a nutshell, now: I'm not sure of the answer to your question, but for now, I'm playing it safe. /images/graemlins/laugh.gif