Set lists are not the best way to approach a Sing-along session unless you have wordsheets. Even if you KNOW the words they KNOW the words differently. A sing-along is fine for the participants who don't frequent folk events regularly, but die-hard folkies well -they do prefer a challenge (by and large).

For sing-a-rounds - prescribed songs ruin the spontineity. And how do you get to be inspired to learn new songs?