I would mark the diagonal if it's for a special quilt. Sew just a hair outside the line to allow for the thread fold over and check before trimming the corner off.You could also draw a line with a sharpie on painters tape and line the line up with the needle and stick it to the bed of the machine. If you start sewing at the corner and keep the other corner aligned with the line, you can get a good result. Do not use painter's tape on the bed of any machine that has decals (like a featherweight) as it can peel them off.