Yea, most printing devices are not 100% true in measurement. Small images, yes, they can be more accurate. The smaller, the more correct it is. The larger you go, the more it might be off.
Draw a 14"x16" box with an outline stroke of .5" and print it. You may find that neither side is truly and exact 14x16" and more likely also find that the length will be off more so than the width will be. I don't know if this comes from the program used or the printer used or both. So making a few seps horizontal and another few vertical is not going to work out well in most cases considering you are more likely doing that for size reasons.