I did not understood your main body question, but regarding "Line ends are ALWAYS round?"
The answer is yes.
I am not an expert, but my comments are:
- there are no mathematical perfect squares in real-life.
- if you consider the mechanical processes they will result on round elements: eg a plot of the smallest point will be round and not square.
If you need to draw a "square" (Eg on the silk screen) you can create a zone filling (I guess it will have the smallest round segments).
If you can also trace very small track (line) segments. They will look to your eyes to be squared.