With JSAM, all access is via the loopback addresses. So, if you specify JSAM access to a server via the IP address, the end user must use the loopback address to access it. that is why the DNS name method is more attractive - JSAM modifies the HOSTS file on the client's PC to map the DNS name to the appropriate loopback address.
I'm not sure if you could use the IP address if you used WSAM to do the port forwarding. I've just never tried it. I am guessing that would work OK.