Port forwarding/open ports?

To be able for another person to connect to my Netmeeting (conf.exe)in Windows XP and share applications I would need to open the ports 1720 and 1503 in my router firewall.

My wonder is how much more vulnerable I will be if I do that? It would be nice not to have to open and close those ports over and over again in my router firewall when I need it and instead having them open all the time so it will just be to start Netmeeting when I need to collaborate and share applications.

- Is it just a risk (bigger or smaller?) when I have Netmeeting started since I suppose some good hacker would need an application that actually listens to those open ports to be able to do anything? If I normally don't have Netmeeting started I suppose the ports could just as well be open in my router all the time?

- If it is a risk as described above, what if I create a rule in my software firewall that blocks those two ports on the computer in my LAN to which the ports are forwarded? It is quicker for me to put that rule on and off in my software firewall than to open and close the ports in the router.

- These same questions above goes for the one port one can choose to have open in the router to give the best possible chances for good sound quality for Skype IP calls. How risky or not is it to have a few ports open in you router firewall?