When I worked at a nightclub, I would play a slow R&B number, as the lights come on. A little bump and grind sort of thing, whatever is hot/recent....right now I would play "The Dream - Rockin that Thang"
Some weddings I have done....I either played a singalong, or a slow song. Sometimes if they had no preference, "the Piano Man", or "Can You Feel the Love Tonight".....if its up to me, and they agree to it, I may announce the last song, and either play an instrumental of the song the bride and groom danced to as their first dance, or the same version....asking the group to form a "circle of love" around the bride and the groom, if they are still there.
Other? I will play whatever it takes for them to realize its over.....something they wont dance to, will not fight to, or something I like.....lol.