While it is true that you never know who you will run into the future, I would still recommend giving feedback.
I'll give one personal example. I ranked University of Washington very highly, but ended up matching up at Kessler, which I ranked #1. I told Dr. Massagli at Washington that the reason I didn't rank them #1 at the time was their relative lack of strength in sports medicine.
I have since met Dr. Massagli on several occasions, and have a pleasant relationship with her. She told me that she valued the feedback, and since then they have expanded their sports medicine program, so that now sports medicine is a strength of their department instead of a weakness. They took what was already a great program and made it even better.
I agree it's not prudent to not make petty comments when giving feedback, but honest feedback can help make programs better.
> How are you getting these surveys.. through e-mail or snail mail? I believe constructive feedback is good as long as our privacy is respected. Sonso is right that in smaller specialties or communities, everyone seems to know everyone else and you never know when you will end up working with someone.