It depends what services you're running on the machine you're port forwarding to.
If you're running Apache or IIS and you port forward port 80, obviously port 80 is exposed to the internet and people from the outside world will be able to connect to the server. This has implications in that you should keep the software patched up to date and be aware of any security vulnerability patches released for the likes of PHPBB hosted on the server.
As with anything, the safest computer is one not connected to the internet at all!