Depends on the speaker setup.
If, like I normally do, I am just using a pair of speakers to fill a room and they are positioned at one end down the dance floor, I use stereo. Things sound better (your music has more space in it – it’s how it was mixed after all) in stereo.
BUT – If I am DJing at a venue that doesn’t use pair’s of speakers down a rectangular shaped room, I use mono. Just to make sure that everyone can hear everything – who knows how it is wired up if it is any more complicated than run Left down one side of the floor and right down the other…
If in doubt though, use mono. It’s the safer option.
bob6397