I don't know about you but over the years I have found no matter how careful I am setting the printing paper in the easel before exposure, it can still come out wrong. I'm talking about making sure the the picture is parallel to the sides of the paper. It is something we all have to deal with at some point. The problem being you can not tell this until the paper has been processed by which time it is to late.
This means that the paper has to be trimmed true again. To allow for this I have increased the margins around the printed area and even the size of the paper. Which I think is a waste as I would prefer to use the whole sheet to print on. But aesthetically speaking I think the margin around the picture adds to the overall effect and stops finger marks getting on the image. Which leads on to another skill that needs to be mastered, the art of trimming your print parallel. At one time it did not matter what I did I could not get it right, that is until a friend introduced me to the cut edge principle. This is where you use the freshly trimmed edge as your straight edge for the next cut and so on round the print. Wow! what a difference it has made over the years.
The knowledge of this method has helped with the trimming of my FB prints that I stick to a pane of glass so they dry flat - described here in another article.