In general, I don't think I would use it as a preventative like they say. first reason is, I don't think it would work. If a disc is going to go, it is going to go. And braces on dogs in general aren't useful. If tight enought to "work". they rub and are uncomfortable. And then the muscles it is supporting become weaker....
Not that having strong back muscles stopped Laika from having a herniated disc but intuitively, it would seem the stronger the muscles surrounding the spine are, the better.
Braces have been discussed on dodger's and commented on by vets and it was all pretty negative. Not that dodger's is the be all and end all of all information but it has been talked about.