No they don't have to be exactly 180 degrees apart, and you also have to consider that the dies could also be misaligned and that misalignment could be in any direction. So you have to start with the images at 180 degrees and centered and then start looking to see if the clashes are "close" and could you make them fit better with a slight rotation, or offtetting the images slightly, or both. It is also possibly the clash marks could come from more than one clashing with the dies in a slightly different orientation between clashes. It can take some practice.