There are other benefits to having the reticle centered.
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A scope should always be mounted so under normal shooting conditions the reticle remains near the optical center. This is especially true of inexpensive scopes. If the reticle is moved excessively off-center, the windage or elevation calibration may become inaccurate. This is why expensive bench rest scopes are custom made so the reticle is centered for a particular rifle mounting and zero range. Competition shooters want their scopes to move the impact point a known distance with each click through a wide range.
The traditional way of mounting a new scope is to first attach the bottom half of the rings to the rifle and with the rifle on a good steady rest, sight through the bore at a target and then through the scope while it is resting on the rings. If the mounts are parallel with the bore then the scope should need only minor adjustment to move the reticle onto the target. More than a few clicks in any direction indicates the mounts need to be shimmed or reshaped. After shimming or shaping the mounts most good smiths will also lap the rings to ensure they are aligned and will not bend the scope tube when tightened. Even slight bending of the tube may cause the scope's adjustment mechanism to bind. Once this is completed, the scope is tightened in place and checked again to verify the scope is still parallel with the bore. Only after proper alignment are the windage and elevation adjustments moved. This method will provide good results, but does not ensure the reticle will remain in the optical center after the arm is zeroed for its trajectory.
An optical boresight comes in real handy on firearms where it is not possible to look through the bore, or to keep the reticle centered when zeroed for normal shooting conditions. But this does not mean simply clicking elevation and windage until the X's line up! If the boresight is new, always first confirm it's grid is near "zero" by checking it on another sighted firearm of the same caliber and type of mounting. Note the point where the reticle falls on the grid. This point is usually a much better indication of the probable "zero" setting than the boresight grid's center or a view through the barrel. As above, if more than a few clicks in any direction are required to put the reticle on the proper grid point, then the mounts may need work.
You can check to see if a scope's reticle is centered by rotating it in the mounts while looking through it at a boresight grid or distant object. If the reticle moves in a large circle relative to the grid or object, then the reticle is not centered and your mounts probably need to be shimmed to keep it centered.
By keeping the reticle close to the optical center you will discover windage and elevation adjustments are more accurate over a greater adjustment range and the scope may seem to shoot better groups. You should especially notice the difference with inexpensive scopes. Any reduction of optical parallax means changes to eye position will have less effect on where the bullet hits. Another benefit is differences in mounting height will have less effect on boresight readings, so it is easier to compare settings between firearms.
http://www.shootingsoftware.com/borescopes.htm
