When I did my head gaskets last year, I was amazed at how clean the tops of the pistons, combustion chambers, and intake ports were. Just a very thin layer of carbon on some surfaces - that was it. The engine had about 40k miles on it. In the ten+ years I've owned it, it has spent a lot of time idling in my garage as I was fiddling with things, so I expected to see a lot of carbon, particularly in the intake ports. I was wrong. I think the stock cam (which has very little overlap) combined with the occassional 25 psi WOT blast does a good job of keeping things clean. Using high-octane fuel (94-octane in MI, 93-octane in Chicago, and occasionally 100-octane for drag racing) probably helps, too, since it usually has more detergents and alcohols in it. I should mention that the inside of the intake manifold also had a thin coating of "lacquer" on it which I attribute to some oil getting sucked in via the PCV system.
For reference - during my days working in the auto industry, we often tested our new engines by running them on dyno durabiliy tests. A typical test would be around 600 to 800 hours. Most of that time, the engine would be at WOT. When we tore-down the engines at the end of test, there would be practically no carbon anywhere - the piston tops, intake ports, valves, etc. would all be nice and clean. Of course, when we would tear-down engines that were returned from the field (i.e. a customer's car or a taxi cab test), the story would be completely different - lots of flaky carbon in the intake ports and on the piston tops. Moral of the story - WOT removes carbon. So, if you use your GN as a daily driver and spend a lot of time cruising around in stop-and-go, you might benefit from some cleaning (I stress "might"). If your car spends not too much time driving around and a higher percentage of time at WOT, then you probably won't see much benefit - it's probably pretty clean in there already.
Hope this helps...