When printing your keys make sure you use a proper font (that don't have similar characters - like "I" and "l", zero and big O).
Also don't trust your printer - some printers have hard drives. And copy of all printed documents is saved on your hard drive (and then "erased" but it still revocable using "undelete" programs) - even in Linux.
You can use this trick to prevent copies of printed pages to be written on the computer's hard drive (this is NOT applicable for printer's hard drive):
mount -t tmpfs -o size=1G tmpfs /var/spool/cups chmod 0710 /var/spool/cups chown root:lp /var/spool/cups mkdir /var/spool/cups/tmp chmod 1770 /var/spool/cups/tmp chown root:lp /var/spool/cups/tmp
Also you need encrypted home folder (/home/yourusername) to be safe.