There's no need to toss your cases early, and there's no reason to EVER
get a headspace separation. You should measure your handloads (when you setup your dies) to see how much chamber clearance YOUR handloads will have (at the shoulder) in YOUR particular chamber.
Bumping the shoulder too far WILL cause thinning of the case body. Headspace separation happens just above the case web, because this part of the case gets stretched when a case is blown forward at each firing. You're right about FL dies - when they're adjusted accurately. Whether you FL or NK size - it's a good idea to push the shoulder back a measured distance.
I developed the Digital Headspace Gauge
to help shooters keep their chamber clearance to a minimum (about - .
002") This gauge provides the easiest way to get this measurement.
Most dies will push the shoulder back quite a bit more than necessary, and some chambers are larger than necessary. The difference can be considerable, so it's best to measure your handloads, and get it right. Most shooters are amazed to see how far off factory ammo is compared to their particular chamber. Check this out on our website ...