Benjamin, when I started printing not to long ago, I got the Book "Way Beyond Monochrome" it had a section on presentation including matting and mounting photos, I also knew about fiber paper and keeping it flat, so I used 2cm on the sides and top. I also learned about non-permanent mounting, and leaving a 5mm edge of the print paper showing and 1.5cm at the bottom to sign the print. Now that I've matted and framed one using a "T" mount of self adhesive linen tape, only on the top of the print to tape the print to the back board I will reduce the sides to 1.5cm, it should still keep the print flat.