A lot of common sense in this post!
I used to work as a gunsmith, and being the new hire my job was sighting in all types of firearms up 460 caliber rifles, .45 caliber handguns and 10 gauge shotguns. When we were busy I would sight in guns up to 12 hours per day, 6 days a week, and 4-5 hours on Sunday. could usually get 6-7 sight ins per hour if there were no gun/mount/sight problems.
The first thing to know is if you can shoot consistent groups of an acceptable size, doesn't matter if they are in the bullseye or not. To many times people have holes scattered all over the target, and say the gun isn't sighted in!
If you can shoot acceptable groups and they are not in the bullseye, you need to look at your shooting style and habits, or adjusting the sights.
If you are comfortable with your shooting style, and have no major habits or errors, I would adjust the sights to suite you and the particular load you are shooting.
The biggest thing I learned in sighting in thousands of guns for others, is that we don't all shoot the same firearm/ammo/sight combination to the same point of impact. Especially when it comes to handguns and shotguns. You can have all combinations of hand/finger sizes, fat/skinny, long/short, big small, etc.. And to expect everyone to grip and pull the trigger the same is not practical.