For small screws, I use the wiring tool with screw cutter and put a nut on short of the cutter, but for 4-5mm and up I allways put 2 or (if room) 3 nuts on it and put it in the lathe 3-jaw. It is very easy to get the right shortening everytime if you have a DRO (Probably without a DRO too). When the 2 or 3 nuts are removed, there is normally no need to clean up the thread. It is also easy to put a nice chamfer on in the lathe.
If you want rounded screw ends, use a radius milling cutter as a turning tool.
Kai