Cleaning the exterior of the tube is not required for regular operation. Dan has weighed in on this a few times. The April discussion shows a picture of one of their test units. So it is mostly cosmetic if you want to clean it. I do clean mine regularly because it does allow me to see the pink glow and have a better view of things to note if something goes wrong.
Lens cleaning after 40 hours or particularly tough runs like cutting Corian or stone which can kick up a lot of dust.