Hi Chris,
Stretching seems to happen along with planishing.
I've seen the stretched panels from some "top shops" out there.
The wheeling machine does stretch when planishing, and I have seen that from some top shops, too.
The pneumatic planisher hammers also stretch while planishing, although with some knack there can also be some shrinking.
The pneumatic hammer I use is fine for planishing without any stretching, and is also fine for shrinking while planishing.
Lots of ways to skin that old cat fish.