You would only need to port forward on the other computers if they also use LW. But if they do, they would need a different port to be port forwarded; ie: one port for each one that uses LW. eg: port 6346 for one, perhaps 42628 or something for the next, etc.
If more than one computer on the network is using LW at one time, then that may have some type of effect also. You might want to consider this patch if you use XP http://www.lvllord.de/