In my opinion the stiffeners are a must; the 350F final fabric shrink pulls on that edge with a LOT of force. Some folks, I have heard, stop at 250F or so, but I have seen several in-flight videos of the top surface of the wing bulging up; so I personally want maximum fabric shrink and tightness on the wings. Even with the stiffeners I put a straight edge on the trailing edges after 350F shrink (I used Polyfiber) and found them slightly bowed inward about 1/8" at midspan, which was not visually noticeable.