2nd Commandment: You shall always default to block on your iptables rules.
If you are configuring iptables (or any firewall for that matter), you should always start with a block all rule. Within iptables, it is very easy to do with:
/sbin/iptables -I INPUT -j DROP -m comment --comment "Blocking all traffic by default"
So that everything in blocked. After that, you can start opening up the ports you need and the IP addresses that need access.