NAT isn't really security, except by a certain kind of obscurity.The internet, and most tools are designed to be used from end to end anyway. I would treat any individual system behind a nat the same way I would treat a system on the open internet.
Its worth considering the different mechanisms of getting ipv6 access, from the least native (Teredo), Tunnels (and there's different protocols that work well in different situations), ipv6rd (essentially an ISP run tunnel, that's a good way to get ipv6 quickly on an existing ipv4 network), to native (We use SLAAC and NDP I believe).
If you're on a less than utterly ancient windows box (XP or better - but I don't have anything worse than a SP3 box, and that's under duress), you probably have the option of non native, teredo support. You might already be on ipv6 and not realising it. Teredo kind of sucks and except in a few situations its worth explicitly turning it off.
Tunnels need a client of some sort, and that's even more work than a native install.
Outside of this ts nearly impossible to set up native ipv6 by accident. Even where your modern router supports it, you need to explictly set it up, and there's 3-4 different mechanisms in common use. My ISP uses ipv6rd and SLAAC on different physical connections, and the instructions are in the equivilent of a filing cabinet in a toilet. The alternative is a tunnel, and that's essentially at least an hour of work.
I would treat any system that's open to the IPV6 networks the same as I would any other system that's on the open internet. If it dosen't need ipv6, turn it off. Its trivial, and I've done this with my XP systems. If it does, make sure its secured. There's very little that absolutely relies on ipv6 in the current transition period that cannot fall back to ipv4. One notable exception is homegroups on windows 7 or later
The good news is most modern OSes with ipv6 support have their own firewalls for IPV6, and you shouldn't have too much trouble locking them down.
IPv6 also has an odd advantage. With ipv4, you often had many exploits that randomly scanned you for open ports. IPv4 NAT mitigates that a little by hiding the clients behind a main ip address. IPv6 mitigates that by having a huge address space it is implausible to completely scan.
At the end of the day NAT is not a security tool - its one meant to solve a very specific issue (the difficulty in assigning public IP addresses), that makes it a TINY bit harder to access a network from outside. In an era of router firmware hacks, and massive botnets, I'd suggest treating any system, ipv4 or 6 as if it was on the open, end to end internet. Lock it down, open up what you need, and not worry as much since you have actual security, rather than a cardboard policeman.