I use a gasket anywhere one was originally used, and most often use Permatex #2 lightly on both sides, just enough to not ooze out. (knock on wood, but so far my new "Driptroit" Diesel hasn't dripped..)
For valve cover gaskets I use contact cement and stick the gasket to the cover, and nothing on the other side. It's stuck in pace and reusable for many valve adjustments.
Some if not all newer rear axles use some type of silicone goop on the housings, I've used it too and it works.
But I still prefer to use a gasket if one was used originally.
Grigg
