Suppose an object with sloped faces that nearly but not quite align with X or Y axes:
I guess a keycap would be a practical example.
And suppose someone wants to finish the sides with two “contour” operations limited to “curves only”, like:
and
Each operation does well for half of the sloped faces while dragging unhelpful marks across the faces that will get treated better by the other operation. Pesky first-world problems 
.
When cutting “curves only”, there is already a “max angle” parameter to exclude nearly vertical faces.
A second limit to exclude nearly parallel faces would help in this case, and might be an “easy”[*] gain for more general utility.
UI? I don’t know. Maybe call the current parameter “max vertical angle” and the new parameter “min horizontal angle”?
[*] apart from the usual * or three