Should students be shown the grading rubric?
Sharing a good rubric with your students can be a helpful way to let them understand what is expected of them.  There is a down side in that the rubric then provides a basis for students to complain that you've graded a paper unfairly.  Don't release the rubric unless you're really willing to give credit according to the rubric even if a paper has obvious flaws in areas not covered by the rubric.