We use sealant on the case halves......but it's not a have to thing at all. We assemble a lot of saws that may have been handled rough. People with drive crap between the case halves to split them, they may have been a victim of shrapnel damage, etc. We've just made using Loctite 518 on the case a part of our routine.
On the Meteor piston.......no worries there. They are a fine piece of work. I've used them rather than OEM in some cases. Even when I had a choice. In certain saws the Meteor is a little wider on the skirts, or has the pins in a little better place.