If you're comfortable making changes to the Postfix configuration, it should be fine. Keep in mind that updates to Zimbra will revert the postfix configuration so you will have to make any recreate any changes to your postfix config manually/programatically.
I have a few custom things in our postfix configuration (mainly to have local mail to other mail servers on campus routed directly instead of via our outbound spam filter) and haven't had any issues with it.
You should be very comfortable with postfix though.
State University of New York at New Paltz