Often departments have guidelines for the numbers of A's, B's, C's, etc. Explain the weighting of exams, quizzes, make-up procedures, etc. Consider grading policies as a contract between you and your students. Contracts should be changed only in unusual circumstances and after reaching consensus with the students to avoid capricious grading charges. Grade and return exams, quizzes, homework assignments, projects and papers as soon as possible.
Students tend to take grades more seriously when assignments are consistently and quickly returned.
If the student is not satisfied, then you can talk about the situation with the head TA, coordinating professor or another experienced faculty member who may have suggestions you haven't considered yet. If a solution cannot be reached, students do have the right to follow departmental guidelines regarding capricious grading. Keep accurate and complete records of all grades. It is important to keep books for several semesters in case questions about student transcripts arise or you are asked to write a recommendation for a student. University students have legal rights to privacy, so social security numbers or student names may not be used when posting grades.