Neck sizing only is fine if using your fired cases in the same rifle every time, and that rifle is either a bolt action or single-shot rifle. Semi-autos and some others (such as slide actions) are far less forgiving in finished cartridge dimensions. Many self-feeders will require "small base" sizing dies to fully resize cases to minimum dimensions in order to function properly.
While most modern rifle manufacturers adhere to SAAMI standard specifications, and finished chamber dimensions are probably far more uniform now than at any time in history, it remains true that each and every chamber (and every other part of every rifle) will vary, hopefully within the normal range of manufacturing tolerances.
Top-of-the-line competition bench-rest shooters may have die sets specifically made for each specific rifle, using either fired cases as a manufacturing guide or doing chamber casts to assure the closest possible match in die sets. The rest of us use standard dies, which will vary from manufacturer to manufacturer, and from one production run to another production run, and adapt our loading procedures to achieve the best possible results for our applications.
I remember a hunting partner years ago, way up in the mountains in the middle of elk season, discovering that his very carefully prepared (neck-sized) handloads would not chamber in his rifle. Fortunately, others in our party were using the same caliber so we could lend him some ammo. Otherwise he would have had a very disappointing hunting experience.
