If, like me, you wish that TextEdit did not show its ruler by default, this hint is for you.

TextEdit displays the ruler by default when creating or opening rich text documents. I find the ruler visually cluttering and distracting, and I rarely ever need it.
Unfortunately, TextEdit does not offer a way to turn off the ruler by default in its Preferences window. But it can be done using the following simple steps:
Quit TextEdit if it is running.
Enter the following command into a Terminal window: defaults write com.apple.TextEdit ShowRuler 0
Open TextEdit.
Enjoy increased visual and mental tranquility.

You can always show the ruler if you need it for something by pressing ⌘R or choosing Format > Text > Show Ruler from TextEdit’s menus.

To revert TextEdit to its default setting, repeat steps 1 to 3 above, but use this command in Terminal instead: defaults delete com.apple.TextEdit ShowRu …

Source: Mac OSX Hints