The Baron said:You can do this the first time you wash a dark item...it is supposed to help keep the dye from bleeding.
I've also heard of pre-soaking clothes in cold water and/or vinegar before washing the first time, to achieve the same effect.
As far as I know, you shouldn't need to do this after the first wash, since by then the dye should have "set".
You can also use salt to distress your jeans, but for that you use coarse rock salt, and a lot more of it (3 or 4 cups).