Teach everything you grade and only grade what you teach; otherwise, you're grading your students on something they've learned somewhere besides your course, whether it's skills and norms they've learned from courses in their concentration or where they happened to go to high school. 

A note on "teaching everything you grade": If it doesn't seem feasible to teach every skill or sub-skill that will go into an assignment, keep in mind that the course syllabus and assignment prompts are important teaching moments in the term. By making general expectations and grading criteria clear to students from the beginning of things, you can also make sure students are able to ask any questions they have and perhaps get support (from instructors, or the Writing Center, or the Learning Lab, or the ARC, or online resources, such as the Harvard Guide to Using Sources) for graded aspects of the course that aren't directly taught in lecture or section. 

A note on "only grading what you teach": Most teachers know what it feels like for a submission deadline to approach faster than we can cover every single element of the assignment as thoroughly as we'd hoped to. Sometimes we have the wiggle room to push back the deadline, but other times we don't. When we don't, the most equitable solution is to consider changes to the rubric and be as transparent as possible with students about any changes. How this looks in practice will vary from situation to situation, but as a general approach it's a matter of treating your rubric like a writer treats the thesis of a draft: it's a best guess about where things are going, but it isn't a static claim; it should—and very likely will—undergo some revision in the face of evidence. 

Even in a perfect world where grades were redundant, narrative feedback would still be necessary for feedback to be meaningful. In fact, a grade without any further feedback isn't especially meaningful. 

A corollary to this principle: students who get an "A" need narrative feedback every bit as much as students who get a lower grade. To be sure, students who get an "A" might be less likely to ask for an explanation of their grade, but that's not the same thing as the student understanding how or why  they succeeded on an assignment and how they might apply what they've learned beyond it. 

When incoming first-year students take the Writing Exam each summer, they answer a series of questions about their experience with academic writing. One of the questions asks them to share any concerns they have about college-level writing, and a very, very common response is this: "I'm worried that each of my professors will be looking for different styles of writing, and I'm not sure they'll like mine." 

It's a reasonable concern on many levels, and it reflects a very common sense—among students and instructors alike—that grading writing is "subjective." It doesn't need to be, though, and it shouldn't be. If you share an assignment prompt with your students at the beginning of a unit, and you discuss it together, and students are aware of how they'll be graded, and you teach them the skills they need and give them opportunities for practice and feedback, and then you give them a grade that is accompanied by comments that describe the relationship between their work and the assignment + rubric—that isn't "subjective." It's highly intersubjective, and a goal of Gen Ed Writes—including the different versions of the assignment decoder exercise—is to offer a clear framework for making the cultures of writing in Gen Ed as intersubjective as possible. 

A corollary to this principle: Creative or non-traditional assignments don't reintroduce the problem of "subjective" grading into things. If anything, their differences from standard academic essays simply make us more aware that we need to step back and think about what we're teaching and how to give students feedback on it. If it's hard to figure out how to grade an assignment, it's worth spending time thinking about its design; if an assignment is well designed, it will be much easier to know how to grade it.