In my opinion hackberry should be heat treated. And since it needs to be tempered,most of that twist will come out over a form while its being heat treated. Kill two birds with one stone.
*edit*...actually you can kill mutliple birds with one stone this way..you can fix twist,side profile issues,induce reflex,string tracking etc...all while tempering it over a form. I've taken nasty 90 degre plus twisted staves with ups n downs,and string tracking off by 2"+,and fixed it all over a form in one tempering session and inducing some reflex n shape.