The major assumption is that the receiver, the rings and the scope outer construction are in any way horizontal or perpendicular to the vertical reticule stadia
They seldom are....
Only one thing can be used to reference the vertical nature of the reticule
That is the reticule itself
Mount the scope loosely in the rings
Shoulder the gun
Line up reticule on the edge of a building
Align
Tighten rings using a diagonal sequence
Go shoot
Unless you are waaaaaaay off or shooting at 1000yds it is unlikely to make any difference