Even as a grad student, my teaching evaluations couldn't negatively affect me. The only people it could have real, negative consequences for were instructors, who work on contracts. As an example, my department head (perhaps unintentionally) sent everyone an email saying any scores of less than 3.73 on a 1-7 scale were grounds for termination of that instructor's contract. I did not envy them at all, basically being highly educated customer service workers.