MY information on this came from doing some vortex flowmeter engineering.

If you stick a strut of relatively unimportant but uniform cross section (in this case, a wing) into a flowstream, it will shed alternating vortex sheets. These are caused by the flow separating at the trailing edge. As the vortices shed they impart a reaction force onto the structure. The frequency of the vortices varies directly with speed:e.g.: twice as fast at 60 as at 30.
Once the vortex frequency hits the resonant frequency of the structure/control system upstream of it, all hell breaks loose and pieces start coming off.
If you want to SEE alternating vortex sheets, go watch a flag. The ripples in the flag are the alternating vortices from the flagpole. In a brisk wind, the flagpole goes into resonance and you hear the flag rop slapping against the pole as it shakes.
In Vortex flowmeters, we shape the strut to MAXIMIZE the force transmitted back to the strut so we can read it electronically and infer flowrate. In airplanes it's important to MINIMIZE the forces from the vortex sheets, so you make the control surface sharp-edged so the mass of the vortex sheet is minimized, or square edged so that when a vortex forms there is no reaction component perpendicular (normal) to the separation surface.

Rounded trailing edges provide a surface for the vortex sheets to push against as they jump off, so the excitation force on the structure is greater.

Kamm's work showed that the blunt afterbody dragged along a separation bubble and the free-stream streamlines were very similar to a fully tailed shape. I extrapolate that to mean that a squared-off aileron would act as if it has a greater chord than actually exists.

IN PRACTICE: Trailing edge shape is probably about 3 orders less important in preventing flutter than having a tight system. As Paul said: Tight systems don't flutter. The obsessive compulsive way to say this is to say that tight systems have a resonant frequency that corresponds to an airspeed greater than terminal dive speed. With a tight system, You can't GO fast enough to get flutter.