When I am making Mixed Media artwork I like to keep a pre-cut Mat "window" nearby to help me isolate parts of my work in progress that may actually be more exciting than if I kept the "whole" piece intact. I have been criticized for giving up too soon on "resolving" a whole piece and I understand that comment. I even at times feel a little guilty for reaching for my mat window, but I also appreciate that I respect my visual eye and if cropping is what is called for....that is what my artistic intuition tells me. Some guidelines that I use when I am making this decision to crop or cut up a piece of work is does the section I have highlighted show more composition integrity and or excitement than the piece as a whole? Sometimes eliminating "noise" or complexity is helpful. Are the color relationships keener in the smaller section chosen? Do I get more drama, serenity, mystery, movement etc. in the portion of the artwork that I have chosen to feature. These are some ways to think about should you crop or not.