g.willikers, that would only matter if the rifle were canted--which always results in horizontal dispersion.
This is one of those "yes..but" answers. Yes the statement above is correct, BUT whether or not the dispersion has any meaningful impact on accuracy.
A canted rifle is of no concern as long as the shooter has good data on what adjustments need to be made for distance. At a High Power match on the 200 yard line nearly all the shooters cant the rifle, and many also cant the rifle in the seated position at 300.
An AR-15 with service rifle sight is about 2.5 inches above bore. The horizontal effect can be estimated by dispersion = 2.5*SIN(angle). So for three degrees of cant you are looking at 0.13" of horizontal dispersion. For small angles there is small dispersion, for large angles there is large dispersion.
In High Power, the X ring is a full minute, and the ten ring two minutes, a tenth of a minute in horizontal dispersion is meaningless most of the time, especially if the shooter knows the sight adjustments to make because they have good dope on their load. So canting the rifle to make it easier to shoot can, and does, result in higher scores in that discipline.
For the 1,000 yard benchrest club, three degrees of cant isn't a good thing at all. The shooter to rifle contact is minimized, so having things perfectly aligned horizontally is a good thing for repeating performance.
Jimro