I agree with bill113 about needing to level the scope, not the action. I do a lot of scope installations because I run scope clinics at two local shooting ranges every month.
It is not uncommon for me to see reticles misaligned to the turret axis by up to three degrees. In this case where you're dialing elevation, the elevation turret axis must be plumb. If you're holding off elevation using a BDC or milling reticle, then the vertical reticle line should be plumb.
For long range shots you absolutely need a properly aligned anti-cant indicator to hold the scope level during the shot.
For 1,000 yd shots even a small cant of 2 degrees can produce a significant aiming error. I recommend reducing the alignment error below 1 degree. To do that, you need levels that are accurate to within 0.5 degree.
Injection molded plastic leveling tools with snap-in spirit levels or sloppy moving parts are not accurate enough for long range. I typically see errors of about 2 degrees. The Wheeler molded plastic levels can be lapped to make them more accurate. The Segway molded plastic tool cannot be made more accurate because the sliding bar flexes and the fit in the plastic part gets looser with time, causing the bar to be out of alignment with the spirit level.
Another windage aiming error can occur in long range shots when 1) the vertical turret axis is not aligned with the center of the rifle bore, and 2) the rifle is zeroed at short range, like 100 yds, and then the elevation is adjusted for a long range shot. If the vertical turret axis is not aligned with the bore, then the windage error will increase as the elevation is increased. This error can also occur when holding off elevation. It can be eliminated by properly aligning the scope with the bore.
To remove this range-dependent error, you need either the EXD tool, or the RingTrue Reticle Alignment Tool that HighPowerOptics sells. The spirit level on the RingTrue tool is guaranteed accurate to within +/- 0.5 degree, so you don't need any other spirit levels.