We have the beautiful Rapunzel wedding dress. It has a sheer overlay, glitter heavy. I first washed in my IL's washer that does not have an agitator, on gentle and it came out perfect. The second time I was a little more carefree and washed it in my washer, has an agitator, on gentle, and I ruined it the sheer layer was shredded. I was pretty sad, especially since it was really a gorgeous dress ( & $$)
My daughter never really noticed the difference but I did/do.
I always soak mine in Oxyclean then rinse and hang dry in the sun. All the black marks come out around the bottom and sometimes spot clean with Shout. Some of our dresses are the deluxe ones from the Disney Store
($149), I do not take any chances of ruining them!