I don't know if this works from Lion onwards, but it does still work in Snow Leopard (it is ancient advice - the date of the post I found this in is 2004!):
In TextEdit is is reasonably easy to change the margins to nearly anything you want-- down to the very edge of the paper. The other people who responded to your question who said it couldn't be done, just don't realize how easy it is.
First, open the file viewing the RTF codes
• Open TextEdit.
• Do a Command-O and look at the Open dialog box.
• Select the file whose margins you want to change.
• Check the "Ignore Rich Text Commands" option box.
• Click "Okay".
Second, change the margins RTF codes
• In the header of the document you will see a line that specifies the margins. This line will probably being this way: "\margl1440". The "marg" refers to "margin", and the letters that follow it stand for the margin sides: "l" = left, "r" = right, "t" = top, "b" = bottom. I believe that 1440 equals 1 inch. So, 20 equals one pixel on a 72 dpi screen.
• Change the numbers to your liking.
• Reopen the document (without checking the "Ignore Rich Text Commands") and "presto": you've got your margins changed. Piece of cake.