The projects in my class are consistently worth 50 OR 100 points. This means I can have a "generic" rubric for the students to be graded on. These rubrics allow me to have a grading system in place that the students understand and gives them some regularity in a normally chaotic classroom.
I love having my students grade themselves and leave me notes regarding their project as it helps me when I grade their work. This also gives me a chance to communicate to them what they need to work on when we move onto the next project.
Having this rubric also keeps me from having to answer the "Why is this my grade" question a million times!