My opinion is that if the board comes pre-covered with waterproofing compound, you only need to tape and proof the joints, and the screw holes.
If it doesn't, I'd tape and proof the joints and the screw holes, as well as doing two coats of WPC on the entire surface.
Alternatively, I'd cover the whole surface with a waterproof membrane, an example of which is
Schluter Kerdi, which I've read some of the inhabitants of these forums use.
It varies though, as I chose system depending on the requirements of the of the job at hand, and what the circumstances are.
In the end, it comes down to what the manufacturer supplied instructions dictate. If they say "go forth and waterproof everything, and his niece", then that's what I do.
Saw a real cool system being used some time ago, which was sprayed onto the substrate. Thick, and perfect fit, kind of like my ex girlfriend
Anyways. Waterproofing is not an art. It's a science. The guys who design waterproofing systems tend to have a good grasp of it. Read their instructions and guidelines. Don't just storm ahead into the unknown without really knowing what you're supposed to do, as it'll return and bite you up the arse one day.
Cheers.