How often do you wonder whether your pictures are hanging on the wall parallel to the ground (i.e. straight)?
We've stood side by side and debated it several times.
All arguments usually end when you place a level on the top edge of your artwork and adjust until the bubble is in the center.
If the painting does not seem straight after the level indicates that it is, you have a much larger problem - your wall is not straight! This happens due to settling and poor construction - more so in older homes.
For those who have not used a level before, he's a link.
The level is a very powerful and useful tool, especially for it's simplicity of use.