In general, HTTPS is a “must have” for all kinds of web appliaction servers. This is mainly because each time you launch a web application in your web browser, the source code of the application is transmitted from server to your computer. If you are using plain HTTP connection, then, theoretically, the source code can be modified/hacked on-the-fly by a third party which has access to your connection (in routers, proxies, etc). Such situation is much less likely when HTTPS connection is in use for loading the web application.
So, returning to the question - it’s better to use HTTPS for PrivMX WebMail, because its source code is safer on-the-fly. If your server is not configured to use HTTPS during installation, you can configure HTTPS later.