Both of the above are correct. It's carrier specific, so one carrier may have that in their FAA approved manual and another won't. Seems that it's usually the Express type carriers that require it - I've never heard it on mainline type carriers - but you folks fly on various airlines a lot more than I do.

While the open shades allow all passengers to see conditions outside before opening an exit, the FA's are required to "assess conditions" and on some express-type aircraft they'd be hard pressed to do so with the shades closed. If you take time to notice, I think every mainline-type plane has a viewport in the doors so the FA responsible for that door in an emergency can assess conditions outside before opening it in an emergency.

Jim