automatic creation of postmaster@<domain.name>
postmaster@<domain.name> is used for services mails, I saw that ISPConfig configuration use it to deliver particular mail failure. In plus is a normal reference to the world for mail domain managing. I think it should be created at mail domain creation time and should not be possible to delete it unless a forwarder is set. In the same way it should not be possible to delete the forwarder if there is no postmaster account.
I am not 100% sure but for such mail (the service one) I have the impression that is always used the primary domain of the server (the one specified in postfix/main.cf as myhostname I guess). Which is not a problem. But once ISPConfig is complete, the install howto/procedure should suggest to use as primary domain name (the one specified at installation time) a "fake" one (like service.mydomain.name) which is used by the sysadmin, so the first client you insert is not bothered but such mail.