Even drawing a thick black line across a sheet of paper and hanging on the wall would show if the focus is off, without the hassle of printing charts (though the suggestion in your link should work quite good as well).
Focus charts provide a scale (for easy comparison) as well as the black line, which ensures that the camera has one thing to focus on.
Adding to Marc Sabatellas comment on AF, the camera just "sees" contrast, and goes after the most prominent one. Especially with wider lenses (such as your 24/1.8), it is quite easy to get multiple "contrasts" in a focus sensor area, and you don't know what the camera will pick.
Last edited by KjetilH; 05-06-2010 at 10:42 AM.
Reason: Darn translation.