I'm a first year grad student, and I'm TAing for a freshman/sophomore level general science course. The professor who is in charge left it up to me whether I want to use a rubric to grade the students' lab reports, and whether I want to distribute it to them if I do. They already have a "suggested outline" that they can follow to write their reports, but it doesn't assign an actual point scale.
How important is it that I use a rubric, as opposed to simply grading based on my "feel" of how well they demonstrated understanding of the material and whether they turned in work that followed or is equivalent to the suggested outline that they were given? I feel like I can be reasonably fair in assigning grades, but I don't want to be inappropriately arbitrary. Is it very unusual for students to be graded in undergrad science labs without a rubric, whether or not they have access to it?