The port forwarding is done to allow the Slingbox to stream in TCP (Top Connection Protocol). The Slingboxes we currently sell are perfectly capable of streaming outside of TCP, although it can potentially cause picture quality and connection issues when streaming the Slingbox outside of your home network. Here's an article to reference on port forwarding (you might already be aware, but too much information never hurts here):
The ports you need for the Slingbox depend on the model of Slingbox you get. The Slingbox M1 and M2 use port 5301-5325. The Slingbox 350 and 500 use 5201-5225. All other models use 5001-5025.
Hope you found this helpful!