I've never, ever read or heard about anyone shooting their 640 loose. Like most people with them, I do most of my shooting with standard pressure .38Spl, and carry it with .38Spl +P or .357Mag.
BTW, I currently have mine loaded with Spear's new .357Mag 135gr SB Gold Dot load: reduced power and low flash, and is very controlable from the 640.