Is it necessarry? Absolutely not. Most people will tell you that it provides better leverage and heelside response. Personally, I've tried them both ways, (aligned and non-aligned) and I prefer them aligned. My reasoning has nothing to do with leverage and everything to do with comfort. I find the bindings to sit more comfortably on my boot when the highbacks are aligned with the heel edge.
^^I agree with the above 2 posters. I find I have a little
more range of motion for tweaking when they are parallel to the edge.