If you regularly use TextEdit, you will notice that TextEdit displays the ruler by default when creating or opening rich text documents. It’s not possible to turn off the ruler by default in Preferences. Offcorse you can You can disable the ruler in TextEdit with the following OS X Terminal command:
defaults write com.apple.TextEdit ShowRuler 0
If you want to use the ruler you can use ⌘R or choosing Format > Text > Show Ruler from the menu.
Revert to default setting:
defaults delete com.apple.TextEdit ShowRuler