Im not the world greatest builders but i tape ever thing. arfs normally get clear tape and if i build them i use the covering that i put on the surface. this came from an old instructor of ours. i know flutter was 1 thing it stopped but what it does is increase the effectiveness of the control surfaces. when the surfaces deflects if there is a gap some air will pass though and not over the surface hence meaning its not working to its fullest. once sealed no air can pass though meaning when deflected it will move 100% of the air flow and work to its fullest. to me now doing 3d when below stall and i throw a surface u need all the aurthrity you can get meaning being sealed will help. I know all so that it is done in some full size planes as i have done some flying and looked at lots. i know for sure the pitts s1s does as i stood at the tail while it was being hand started and took note that it had a black rubber seal on the hinge line of the rudder and elevators. so its not just us rc guys that do it.