In the overall of precision shooting it's a 1% type of thing. Ideally the vertical crosshair should be aligned with the line between the bore of the rifle and the center of the scope. To be aligned visually is sufficient.

IMO tools that do this for you are of extremely minimal value.

The shooter needs to be aware that all shooting must be done with the vertical hair visually perpendicular to the surface of the earth and that "canting" the gun even 10 degrees can have adverse effects on point of impact. Visual is good enough however.