The lindy hop is largely done on the toes, especially at moderate-to-high tempos. If you are letting the foot fall backwards, the heel is not impermissible but if the rockstep is more like a suicide kick (that is, your intention is to make some brisk turning), then it is difficult to get the heel in.
I just took another beginning swing class last night before swing night at Secret Society here in Portland.
The young woman who taught identified what we were doing as East Coast Swing, but it was clear to that she mostly does Lindy Hop.
And she said that basically, if you go back on the heel, One thing that it does is move your center away from you partner. And you don't want that.