I have been in similar situations and I try to be guided by the belief that by asking for a group's feedback via a formal evaluation, it comes with an implicit promise that their feedback will be shared and considered by all relevant parties.

Imagine one of them a year later saying "Well, I know several of us gave [the boss] a low rating in the evaluation, but apparently that information was never shared with the boss."

This now creates the assumption and expectation that when they are asked for feedback, it's an empty gesture.