tcp ports are dictated by IANA So if you decide to "secure" port 25, nobody will be able to connect using ftp.
This link has 140 pages, just read a few.
Service Name and Transport Protocol Port Number Registry
My point is: you can break all kinds of services without knowing why. And if someone attacks a random port and there is no service behind it to respond, the attacks fails. So no need to block it. This is why attacks go for a lot of known ports. So known ports may require a minor tweak.
Are you trying to harden your box for a reason? A lot of linux boxes have special apps to help you. What OS and version of it do you have?
will show that, so please post it.
Edit: It is not uncommon to harden a UNIX and break some applications.