Yea, yea . . . I know about the "little rings" . . . that's why I clean 'em when I'm done shooting.
This is something I wonder about too. How much shooting do you need to do before you see a problem? I don't shoot a lot of 357 ammo, and I make a minimum effort to clean my guns, but when I do load 357's, they drop right in.
Seriously, I'm not saying it doesn't happen, but I've never seen it.