After you ride in them, they sink down more. I've never had an issue with one resting on the withers, but rather the very middle of the saddle will pressure on a horses back. So just to check it after a couple rides to make sure the middle isn't bearing down.
Other than that, I love mine, but, it only fits the drafts I ride, had to buy something else for the AQHAs, warmbloods and such..