Don't add other repos unless you NEED something that the Centos repos don't have. You can easily break your system.
you can VERY easily brake the system
rpmforge is 100%safe
BUT rpmforge-extra IS NOT SAFE
the *-extra brakes compatibility with red hat
so install and SET UP "yum-priorities" ( READ the documentation on it !!! )
remi ??? unknown i install firefox by hand
epel and EL should be fine for most cases but USE "yum-priorities"