With cedar shafts, I tend to have the best luck just using my hands. For a really crooked spot, I rub the high spot with my palm until friction warms it up. Then I check it. If the heat alone didn't do it, I'll tweak it by bending while the shaft is still warm. I've gotten some pretty ugly ones straight this way.
My experience has been the same as Bud's with pressure...flat spots. I think cedar is especially susceptible to that. Hardwood shafts may be a different story, but I haven't tried any of those.