Well, if you are running ATS as a reverse proxy for a web site, you are likely to want it to be port 80. That is the default port for all Web browsers, for non TLS traffic (plain text). For TLS (HTTPS) the default port is 443.
What does this mean? It means a URL without an explicit port number will default to either of these ports. So,
http://www.example.com/ is defaulted to port 80
https://www.example.com/ is default to port 443
If you can control the URLs, you can make the port anything you like. For example, http://www.example.com:3333/. Note that many ports are already assigned, you should check the register.
Finally, ATS will start up fine with privileged ports, but if you do, you must start the init script as user "root". It will bind the port 80, and then give up its root privileges, running as the unprivileged "nobody" user (by default).