I don't see how it would matter one way or another with a non-tapered control surface. Squared or rounded, they are all aerodynamically dirty anyway. Power planes can get away with this because you have a big ol' tractor on the nose to overcome all that drag. Can't see how squaring off would be any more flutter prone.

Having come from a sailplane background, I just can't stand these slabs of wood they pass off as powerplane control surfaces. The ideal trailing edge, control surface or not, is as sharp as you can practically get. It is the most aerodynamically effcient and leaves the least amount of wake and turbulence behind the wing.