It's really not a matter of speed, nor is it a matter of flutter. A sealed gap makes for a much better control surface.

When a surface is deflected (lets say downward) it creates a high pressure area under the surface and a low pressure area (or partial vacuum) above.

If the gap is not sealed, the air in the high pressure area gets sucked through to the other side. This not only decreases the effectiveness of the control surface due to lack of air control, but also due to the turbulence created by the air passing through. Now, factor in the fact that the gaps may be different on one aileron or elevator as opposed to the other (in fact, you can count on them NOT being identical), and you can see what havoc an unsealed gap can create. Sealing the gap keeps the air on the side it belongs. And regardless of speed, it will make your controls a lot smoother and more responsive.