Before adjusting the AF settings, you really should make sure that there's something wrong in the first place. Many shots that are slightly out of focus are so because the AF picked up on the wrong thing (e.g. eyebrows instead of eyes). It's very difficult to judge from an ordinary picture whether the AF is maladjusted, or whether it just focused on the wrong (part of the) subject. Heck, it's even difficult to judge AF when you're setting up a shot to specifically test AF accuracy.
AF sensors aren't points, but areas in themselves. When they are aimed at an object under an angle, they don't pick the center of the AF spot, but rather the edge where the subject is closest. That's one of the reasons that so many of the so called focus accuracy shots where you see a ruler under an angle have the DOF extend only in front of the claimed center of focus instead of 1/3 in front, 2/3 to the back. It's operator error, not camera error.
To test accuracy, you need to make sure that the AF sensor completely covers an planar object exactly perpendicular to the lens axis. Any object at another angle (e.g. a slanted ruler to see where the focus falls) should be entirely away from the complete extent of the AF sensor. Only if you find an error then, should you start calibrating the AF.
