IMO as not-a-gunwriter, and definitely in the category of picking nits, issues surrounding scope mounting relative to the shooter and rifle are most important relative to getting a well aimed shot off quickly in the field from a standing position.
With this being the goal, regarding "cant" I agree with Iclimb. The issue is achieving consistency consistent with minimizing time-to-well-aimed trigger pull.
First, note that with most people with most factory rifles, the action of a rifle will be somewhat canted when mounted quickly to its most comfortable position on the shoulder. Canted or not, this position is that most easily repeatable with haste and most amenable to being locked into muscle memory.
The only way to insure the action is uncanted in this position is to fit the stock to the shooter to compensate. In my case, about 1/2" of cast-off with an additional 1/2" of toe-out cant in the butt stock trues the action of a quickly mounted rifle to gravity. Most of us most of the time make do with a rifle not fit with precision to our body. This means it most likely will be canted to some degree when quickly mounted to its most comfortable position.
Second, human eye is really, really good, at judging verticality of a straight line and does so unconsciously. If you doubt this claim, tilt a hung picture slightly and watch how people who may enter the room react. Rotate a scope slightly in its mounts and you create a powerful unconscious force that will cause the rifle to be canted by the shooter until the reticle is true with gravity. If this eye-induced rifle cant is at odds with any body-fit-induced cant (as it is likely to be if the scope reticle is blindly trued to the bore), achieving repeatable aim will be more difficult and take longer.
IMO, the rifle should be allowed to fit the body the way it most naturally, cant or no cant. The scope should then be trued (rotated in the mounts) until it is perceived as vertical. This makes the info from the eyes conform to info from the muscles. I think all this allows a practiced shooter to get an aimed shot off about 1/2 second quicker than otherwise might be the case.
