If you look in the notebook source, in \$SAGE_ROOT/devel/sagenb-main/sagenb/notebook/twist.py, search for "last_edited". It should bring you to line 1593 or so. Change 'last_edited' to whatever you want the default sort order to be. The possibilities are given in the sort_worksheet function in notebook.py (last function in the file) as 'last_edited', 'owner', 'rating', or 'name'