When you say flex, do you mean the material your motors is attachted to may flex/bend so that the prop is not parallel with the ground? I know that in the past, I've had aluminum arms that took some hard landings, so the body of the copter was sagging and the props all tilted in somewhat. This caused a yawing issue where the copter wouldn't keep it's orientation. It could have also had something to do with the gyros on the board from all of the crashes. I'm not 100% sure.
Generally, the material in frame arms is pretty solid and reinforced though, so I don't think you want any flex.
A lot of people will 3D print or cut out frame bodies and buy the flamewheel arms to use with their custom frame. That may be an option.
