Comment 1 for bug 430411

It does just work. See the instructions in /usr/share/doc/spamass-milter/README.Debian:

--------------------------------------------------------------
  Integrating spamass-milter into your postfix installation
--------------------------------------------------------------

You need to edit /etc/postfix/main.cf and configure smptpd_milters so
that postfix connects to the spamass-milter socket. Something like:

  # spamass-milter configuration
  smtpd_milters = unix:/var/spool/postfix/spamass/spamass.sock

should work. Note, however, if you're using a chrooted version of
postfix, you'll need the local path to the socket inside of the
chroot. In recent versions of Debian the following should work:

  # spamass-milter configuration
  smtpd_milters = unix:/spamass/spamass.sock
  # milter macros useful for spamass-milter
  milter_connect_macros = j {daemon_name} v {if_name} _

See http://www.postfix.org/MILTER_README.html or
/usr/share/doc/postfix/MILTER_README.gz (in postfix-doc) for
information on how to set tempfail and the various timeouts that the
sendmail configuration above uses.

The defaults for spamass-milter adjust themselves so that no
configuration in /etc/default/spamass-milter should be required.
[However, if you are not doing so, see below.]