I would like LibreOffice to save my documents as a .docx instead of .odt per default.
Yep that works!
It does work. I have tried on Libre Office I have in Fedora 16.
did the steps provided by Jeffery work for you? Please let us know. Thank you!
Go to Tools -> Options -> Load/Save (Expand) -> General -> Always Save As: (bottom right) change to "Microsoft Word 2007/2010 XML".