Pay attention to the uniqueness and anonymous questionnaire. If this does not imply you profile fields for your name, it does not mean that it is anonymous. Most dodgy bosses make subtle differences in each instance (in particular, this technique is effective in small groups), and every anonymity is lost. Therefore selflessly to write "everything you think" - is not always the best idea.
If the survey is not anonymous, it should not be given only positive responses. As a rule, most of the staff replied that "everything is fine", but this is not the best way. On the contrary, filling out a form quite critical (but not fold the stick), you will stand out sharply against the general background and interest the head. Giving answers of this kind, you put yourself as an honest, brave and principled: besides, you can always explain their behavior that "care about the future of the team" and "criticize those things that really should change."