Neck sizing with a full length die still moves the shoulder slightly. This is caused by the expander ball being pulled back through the neck. In most rifles this isnt much of an issue, unless you have a really tight chamber. You can eek a bit more accuracy from using a true neck sizing die. I personally use a neck die with all my rifles. I like to work the brass as little as possible and like the idea of not having to lube my cases to resize.