While using TextEdit in any version of Mac OS X, you can customize your page margins within the document.
However, if you use the Page Setup dialog sheet to change to a custom page size that has different margins or to define (via the Manage Custom Sizes choice in the Paper Size pop-up menu) a custom page size, the document will not appear to use the custom size's margins.
To make sure your document will look the way you expect when it prints, preview it before printing.
This document will be updated as more information becomes available.