I think it depends on if the surveys are pegged to a specific individual. I work in tech support, and surveys can get mailed out (every caller has a chance of receiving a survey if they have provided an email address). These surveys are tied to cases we create. I am partially paid based on my performance. If I get a negative survey, it impacts my pay. Sometimes I've deserved those negative surveys, other times, the customer gives me a negative survey because I can't magically make a replacement part appear in their hands (they have a chance to leave comments, and the comments have been on the lines of I loved the agent, but the policies for repairs/replacements are awful, so it's not just me making an interesting assumption. There are other surveys I have received where the response seems unfair, but not as clear cut as that).
So, if it's linkable to a particular person, I would err on the side of generosity. If it isn't linkable to a particular person, I'd be far more willing to be honest. Also, if you do say something negative, explain as thoroughly as possible. I absolutely hate getting a negative survey with no comments, because then I don't know why my service wasn't adequate, and it's harder to figure out where I need to improve.