What I have found, especially with sourwood shafts, is that even after drying for a year or more, when you apply heat to straighten them for the final time, you will have to completely re work the shaft. Once you get them straightened over several times, there will come a time where they stay straight. I heat mine, straighten them, let them rehydrate for a day or so, and check again. At some point, they will be as straight as you left them and mine then stay straight. I have never been able to heat straighten a shoot shaft and it stay the first time of heating it. JMHO.