There's a minimum trim length included in all published loading data, to achieve consistency between each loaded round, brass should always be the same length, trimming to the min length accomplishes that as well as making sure the brass isn't too long for the chamber which can increase pressure. But what you may be thinking of is some folks trim Maxi brass to shoot in a 357Mag barrel that hasn't been rechambered to Maxi, many 357Mag chambers will accept trimmed Maxi brass without rechambering, most will accept 360 Dan Wesson without any trimming.
Tim