I've never quite understood the reason behind aligning the brushes. It seems to me that if the brushes are slightly cocked, it will adversely affect performance at first, but once the brushes wear down and seat themselves, it won't make much difference if they are off a couple mils in either direction. Additionally, if the brush were at a slight angle, it would give more contact area. While it might cause a little bit more friction, it would also yeild more area for current to flow through.
I'm probably dead wrong on this, but if anyone can explain why, I'd be interested to hear their explaination.