Shapes vs Lines
Ultimately the elements of your model will be filled with color or a pattern. A shape must be connected at all points and closed in order to be filled with a color or pattern. Also, the weld operation (commonly used when producing a complex shape from simpler elements) only can be applied to a shape. For these reasons, your design should use shapes rather than individual lines. The point edit manipulation can be used to alter a simple shape, or a closed shape can be constructed using that specific tool. The line tool should be avoided except for fold lines, measuring, making guides, or for producing non-cut graphic elements.
In this example, the cut line for the element on the left should be one shape (middle) rather than separate lines (right).