Just had to weigh in on this issue
Just look at SANS or CERT, IETF et al and their recommendations for
filtering i.e. egress filters should only allow your registered address
space and/or networks which you allow to transit your network and ingress
filters which filter ALL RFC1918 traffic at your network border. See RFC's
2267,2827 & 3013 Ingress/egress filtering is simply good operational
practice which makes the internet a more friendly place