It does not seem to be necessary but does it help to use port forwarding for pooled mining? I mean forwarding the same port as the pool port on the incoming side of my router to the rig.
When the miner needs work, it contacts the pool. So the miner makes the connection.
The only time the pool needs to contact the miner is when a new block is found, to invalidate existing work units so the miner doesn't waste time finishing a work unit when the block wouldn't be accepted anyway. But this case is handled with long polling -- the miner keeps an open outbound connection to the pool manager at all times to receive this notification.
So there is no need to forward the port, and such a forward would never be used anyway. The mining program isn't listening for inbound connections.
Port negotiations for mining occur via the same protocols as connecting to a web server, which means the port on which your system listens for return data is not the same as the port the pool is listening on, nor is it the same port every time. Forwarding the pool's port to a single machine will do absolutely nothing.