thanks for your answers. to 1: yes, but if you don't enter a 6 characters passwd, it seems that cups don't work, and that you don't have the possibility to change it back; that was one of my mistake at the begining, and when i tried to change the tc or root passwd again, it has had no impact on cups, i.e. it was impossible to register the printer because cups rejected the passwd;
to 2. my experience is that i had to do these even if the printer was configured as default in cups (typically for sylpheed as well as for xpdf).