I say if the criticism is constructive and meant to assist the author, then leave whatever you want. We can't all expect to have a slew of perfect reviews all the time, not even in fic fests. I also think that if you can find something negative about a fic, you can also find something positive. If the feedback is dealt with in a polite, constructive, non-hurtful way, then I think it's fine to leave a little negative feedback.