It depends on which port it is. When I bought a Linksys router it had
the effect of changing my port 113 (Ident) to "closed" from Stealth.
There are directions on www.grc.com to fix this problem. I can confirm
that they work.
If it is another port then I think it is a matter of figuring out
whether you need the service that is using that port - or not.