Guard should be used ROUGHLY on cooldown, at least in my books. It's nice for helping out with medium-to-large hits so if you know a predictable spike is coming but you don't need a large cooldown for it, you can save Guard for it but overall I'd try and use it fairly liberally just to make life slightly easier.
And as Derpette said, cooldowns in general are best used for large predicable spikes. If you're getting down to dangerous levels of health when you aren't supposed to be taking high damage then there's something fundamentally wrong.