Exactly! Ever seen that vid of the Tacoma Narrows Bridge from the 40's? They built the one that fell totally symetrical. On the next one they pulled the suspension cables out of symmetry somehting like 3%. That was enough to solve the problem

It's not a total assurance of non flutter but any mass balance or control horn mounts should never be at any of the points on the surface that may support any even harmonics of lower levels. This means the ends, half and quarter length points should be avoided. Just pick one of those points and move the horn or balance by another 5 % one way or the other.