It's not a bad idea to seal hinge gaps on speed planes. Like it has been mentioned here it will aid in control but as for preventing flutter it is a misconception (unless there is a really big gap and then you probably have other problems in construction as well). Air flowing through the gap in a turn will decrease the effectivness of the control surface. The harmonic that causes flutter isn't from the gap but more likely started from slop in the control linkage that allows the surface to start oscilating.