Vertical alignment of Rifle and Scope?

[Off in the Bushes]

With out hijacking my previous thread.
Ok, so I think that I have enough info to mount the scope. There is a confusing part of aligning the the vertical hold of the cross hairs with the vertical hold of the rifle. All the tools I see like wheeler level-level-level are all basing the vertical hold of the rifle off attached bases which may not be correct. How does one find the vertical centre of the rifle if the receiver is rounded?
Is this accuracy as critical or more then as having scope properly aligned.
Are dovetail levels accurate?
Thanks again I want to do this right and do it once.