Crown Molding

Yes to seal the envelope, other wise air will leak into the attic.
But you can get away with just one coat of mud.

I appreciate the fast response and understand what you are saying but prior to hanging my drywall I added a 6 mil vapor barrier that came down about 6 inches onto the walls where I glued it to the top plates.

I will still proably add the mud and tape anyways, just curious if I had too.

Yes,you do---that single layer of tape and mud is called 'fire tape' and will slow down the advance of a fire into the plastic you used as a vapor barrier.