The clamp is absolutely necessary, in my opinion. The second concern is over bend and needing to bend back. Care and careful measuring after each bend attempt will help to guard against this. Finally, equal bends on both sides are important to keep things running true to the centerline of the bicycle. Once drop to drop distance is achieved, you must align the drops, ensuring that they are bang on parallel to one another. Fingers crossed is a rarely mentioned part of this procedure.