The informal rule for anything a friend (even a business friend) puts out there in public was passed along by Lehman Engel to BMI workshoppers, and I think it was Maury Yeston who jocularly codified it as "The seven-block rule." Since you never know who at the event is another friend, and these decades producers send younger people from their office who tell them whether to request a full script and score, you can't know who the friends are and who can help the show. Maury described it as no matter what you thought of it, anything you say within seven blocks of where it took place will not be negative. And if you observe it, most likely the favor will be returned to you.
Your friends are there to tell the truth if they are enthusiastic, stay quiet or lie if they don't like it, and send theirs note quietly.
There's a story somewhere where Sondheim tells Jason Robert Brown something similar, only positive the day of the performance; then, save the notes for the next day when asked.
