Rather enable UPnP much safer and doesn't leave unnecessary ports open, this will automaticly open ports that are needed and close them when not needed
UPnP has advantages and disadvantages. It can make life easier because you don't have to set up port forwarding since the ports are forwarded automatically for you (if the router supports UPnP). UPnP is therefore mostly aimed at convenience.
However, it's a security risk because if UPnP is turned on in the router, anyone can use an UPnP application and open any ports that they want to without them ever logging into the router. Since UPnP in a router allows Windows to open and close ports on the router as needed, some programs like MSN Messenger take advantage of this for some features since the ports remain closed on the router until Messenger needs them.
However, the disadvantage is if e.g. malware does get into your computer it could open up ports on your router at will to allow malicious traffic to pass through. If any machine on the LAN gets infected, that virus/worm could send UPnP requests to the router to open up all kinds of holes from outside. I prefer to avoid that and turn UPnP off.
Yes, if you do manual port forwarding that port remains open but the way those open ports are normally used to gain entry is to use some weakness of the *application* that is connected to that port. If no application is connected to that port, all communications to the port disappears. Open ports that are subject to attack are usually ports that are regularly used standard ports used by e.g. gaming etc. But since you can use a random port in uTorrent, the risk is reduced.
It's a trade off; either know 1 or 2 ports that are open or have no control if using UPnP. I prefer the former, others the latter. I tend to disable my port forwarding firewall rules in my router once I'm done downloading so then the port is closed anyway.
