With the WSM,most of the time you don't have to use it! Plus I keep a thermometer stuck thru one of the holes.
I use the top vent more on the Performer than the WSM. It's a quick way to knock down a flareup.
Ideally you want fresh smoke to flavour the food, by closing/partially closing the top vent the smoke billows around and ends up giving the food an acrid bitter taste.