There are 2 aspects of "sendmail" (or equivalent). Ability to send local mail to local users like cron job output to root and ability to act as a "mail server" so outside people can send mail to users in your domain.
in /etc/rc.conf
sendmail_enable="NONE"
means it's completely disabled, I'm not sure if that that option in the installer does that or something else.
If you are going to use Postfix, I think disabling sendmail completely is the correct answer, and use whatever Postfix knobs to enable it.
