Don't know if anyone's seen this yet, but KVV + RCS Build Aid = a wonderful way of lining up rotors to COM

[B-STRK]

What's going on: having KVV and RCS Build Aid active at the same time draws the standard coordinate axes (longitudinal, lateral, vertical) through the COM, in both the screencap previewer and the build scene. You can use the previewer in KVV and watch as you manipulate whatever part you need to manipulate to move the rotors about: the preview and build scene will update the axial lines as the COM moves live as you move the part/s as well. Orthographic view in KVV squashes everything in the previewer into a 2D plane perpendicular to the viewpoint, which greatly assists in getting things lined up. 

Please note that the lines will only draw through the COM, despite RCS Build Aid being able to track the ACOM and DCOM. 

For quadcopters/tandem birotor craft, the crossmarks through the axes can be used to gauge distance from the COM to balance things out without using differential thrust. 

I am not sure if having Hangar Grid is essential in getting the lines to draw (as can be seen in my mod toolbar), as it draws its own guide lines for part rotational assistance.