I only have one mounted print which I cropped, and I remember being disappointed in myself when I did it because I realized that it was a failure of vision. I should have seen it on the groundglass.
However, both Michael and Paula have seen it and advised me to keep it in my body of work. I'm very pleased with the end result.
Walker Evans is said to have cropped his 8x10 negatives with a pair of scissors when he wanted to force magazine art directors to more closely follow his vision. The same source (Walker Evans at Work, ISBN 0500273049) said that sometimes his use of the scissors was a bit sloppy, corners failing to meet at 90 degrees from time to time.