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?