Technically you only need to seal one side to make them not leak. But if you only do the bottoms, air can push through the top and push the covering out and it can peal off from my experience. On my giant scale planes I seal both sides so they don't "blow out". I also like to seal the rudder. I use my rudder as much as or more than any other control surface.

BTW, clear Monocoat works great on any color. But solid colors hide imperfections in the hinge job (gap-o-rama).