You don't even need something like Tails. You just need encrypted disk images, and to keep them unmounted when you're not actively using them. This is a capability that is for instance built into Mac OS X.
Well, just as a defense-in-depth, in case of a Firefox 0-day (or 90-day) or something, it'd minimize a leak, slightly?
Edit: Oh, also, since compulsion to reveal keys might be an issue, it's better if there's nothing around. In the US it seems to be if the prosecution can convince a judge that they really did see evidence but you since re-encrypted. I wouldn't want to rely on a judge in that case. Although at this point one is probably cooked but still.
What someone in that situation really needs is the opsec equivalent of a trap door function. Does such a thing exist that is easy to defend and difficult to attack? Could be very useful for e.g. rights activists in remote places of the world.
