You can get away with neck sizing with milder loads, but I've always had to push the shoulder back a tad to be able to fully chamber a near max loaded fired case, there's just too much action flex in the high pressure chamberings. A simple test will tell you if you need to FL size, try to rechamber a fired case, if it will chamber and fully lock up which pretty much requires the round be flush with the chamber face when checked with a straight edge, you're probably good to go, this is usually the situation for a stuck case on an ejector barrel tho, just depends on the load.
Tim