Accurate print positioning is as critical as perfect halftone execution (maybe more so) when it comes to achieving desired results.  This order was returned a couple of years ago due to incorrect (off-center) positioning.  It may not seem like a big deal, but to a designer, this is highly critical.  It was a simple slip in the art department (the graphic was incorrectly placed on the artboard) that went unoticed during first print approval.  We now include a visual centerline on our e-proof which shows the precise positioning of an off-placement graphic.