Depends. Some builds of Tomato (Toastman's for sure) put a firewall up on IPv6 by default. Asus's firmware does NOT firewall IPv6 at all. If you have shell access to your router, I suggest putting up a firewall on IPv6. The following should work (change br0 to the bridged LAN interface and eth0 to the WAN interface, sometimes it's a vlan):

  ip6tables -A FORWARD -m state --state RELATED,ESTABLISHED -j ACCEPT
  ip6tables -A FORWARD -i eth0 -o br0 -p all -j DROP
  ip6tables -A FORWARD -i br0 -j ACCEPT
  ip6tables -A FORWARD -o br0 -j ACCEPT
  ip6tables -A FORWARD -j DROP

Of course insert whatever open ports you want after the first line.