No, you don't need to do that...it's optional. I think that field might be there for those that don't want to forward their passive port range through the router?
Anyhow, I don't have that box checked, but I do forward the 55536-55663 range to the internal address of my DNS-323 (in addition to forwarding the main, active FTP port, in my case 1023, to the DNS-323's internal port of 21), and I also use a dynamic DNS name to connect to the DNS-323 externally, and it works just fine.