Covering the hinge line is not a "Must Do" thing. I used to do it on ALL of my planes. Now I rarely do it.

It is most helpful if you have a large gap between the control surface and the Wing, Stab, or Fin.

All you have to do is to put a strip of covering or tape down inside the "V" (or bevel) so it will seal the gap, but still allow full movement of the surface.

Start by deflecting the surface as far as it will go, then add the covering or tape to the open side of the hinge line. Be sure to get it as deep as you can to avoid hindering the movement.