We had a discussion internally, and since we use the assign feature a lot, some folks came to an agreement:
The assign note should be related to what to do for the assignment e.g. “Please do X this way, then inform someone about it”. This way, if we change the assignee, the next assignee will still get the same note on what they need to do. Our folks also mentioned there may be tendencies to put non-assignment related comments into the note, rather than a separate post, so a little discipline is needed there.
The problem with clearing the notes is sometimes we have the original assigner writing details about what to do for the assignment. And since there may be many posts in between re-assignments, it’s troublesome to scroll back up to find out what the note was so that they can re-insert it if we cleared it automatically.