I'd like to second the comment by Sensai, about the advantage of a collet type crimp die. I do like the Lee Factory Crimp die. I have experienced two problems when using a taper crimp on 5.56mm (.223Rem) and 7.62mm (.308Win). First, if too much crimp is applied, the neck can bulge out and cause feeding problems if the rifle has a tight throat. Second, as Sensai said, this can also result is less bullet tension than if the crimp was omitted altogether. I've actually encountered a situation where the bullet could be spun by hand in the loaded case, held in place only by the crimp, with all case neck tension removed by the bulge in the neck area.
I know, someone is going to say "you're not doing it right". My answer is that I've loaded many thousands of rounds of 5.56mm and 7.62mm for gas operated semiautos, and that's given me an opportunity to see just about everything EXCEPT any serious problem. Rounds that don't meet muster get broken down and reloaded. I do think the Lee FCD is the neatest thing since sliced bread. And I've tried just about every brand of die available.