Hahaha...If you have two verts, you can have it connect them with a straight line by pressing F in Blender 3D.

And yes, once a model is no longer aligned with the xyz axis, it becomes difficult to get a straight line (as far as I know) that lines up with an off-angle mesh.
I usually do all my component modeling while still aligned with the XYZ axis and use the modes that "snap" the object, line or verticie to the desired axis. Example: You extrude the side of a square and press X to make it move only along the X axis. Or grab a vert, line or face (G) and then press Y to move it along the Y axis.
If I need to rotate the mesh out of alignment, I try to do so using numbered degrees so I can rotate it back to the axis if further changes are necessary.
LHammonds