Why do we advise against frames and borders?

Why is my frame or border disproportionate on the printed material? Sometimes people use a frame/border because it is part of the design.

However, we advise against this because there is a chance that the frame will not be in proportion on all sides after the trimming. The borders in your artwork will make even just a few mm's of difference in the cut very noticeable. If you want a border nevertheless, this is entirely at your own risk.