The solution to How to fix TikZ corners in 3D was to use line join=round instead of line join=miter. But I am trying to understand why that is not the default. Under what circumstances would "line join=miter" come in handy?
In the TikZ manual it seems that miter option is useful for sharp corners, but the figure I was having problems with had sharp corners. Or is the rule of thumb: use miter option for sharp corners in 2D diagrams, but round for 3D?
The miter option is more "correct" in that it doesn't add new corners to a shape or introduce rounding. In some cases with very sharp angles, the results might be undesirable though, as in the question you linked to. Consider a right angle with different join types: These could be corners of a rectangle. Only the standard miter option would be what you would intuitively expect.
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[line width=10pt]
\draw [line join=miter] (0,0) -- (0,1) -- (1,1) node [pos=0.5,label=above:miter] {};
\draw [line join=bevel,xshift=1.5cm] (0,0) -- (0,1) -- (1,1) node [pos=0.5,label=above:bevel] {};
\draw [line join=round,xshift=3cm] (0,0) -- (0,1) -- (1,1)node [pos=0.5,label=above:round] {};
\end{tikzpicture}
\end{document}
