In every engine that I've ever owned, in which I've used a good-quality synthetic oil changed at the factory recommended service intervals, I've never observed any significant sludge or deposit build-up under the valve cover.

Short trips that never allow the engine to warm up, and driving patterns that involve puttering around barely above idle speed and only for short trips, and inappropriately using cheap non-synthetic oil in a hot-running application that ought to be using synthetic and then not changing it often enough, are the usual causes of sludge build-up. If that's not your driving pattern and that's not your (negligent) maintenance habit, don't worry about it and don't try to do anything to remove sludge that probably isn't even there to begin with.