Originally posted by Mike James
whatever pressure differential exists on the flying surface will tend to "suck upward" through the gap.
Yes... If the control surface is deflected downward.
However, if the CS is deflected UPward, the pressure will force air downward through the gap.
But really, there is still no need to do both sides. The pressure is not going to be so great that it will blow a piece of tape or covering off (unless it's not attached well).
If you feel more comfortable sealing top and bottom, go right ahead, but it would be like putting two Band-Aids on the same cut.