You should change this parameter only if it is absolutely necessary to maintain root privilege on your NFS mounted file systems and you have checked each NFS client to be sure that it requests a privileged port. If there are any clients requesting non-privileged ports, they will be unable to mount the file systems.
Additionally, changing the value of nfs_portmon to 1 can give a false sense of security. A process must have root privilege in order to bind to a privileged port, but a single ''insecure'' machine compromises the security of this privilege check.