For me it depends on the speed of the dance and the amount of ambient light. For the first dance I tend to use slow shutter speeds to get the ambient light have an impact and rely on the flash speed to freeze the dancers. If the ambient light is low and distance to the dancing couple is restricted ie. they are too far away, I use an LED video light to help out a bit. Sometimes the ambient light is too high, I am not quickly pleased, shooting in that case without a flash and illuminating the couple with an LED video light combined with an increased ISO would do the trick. I use a Yongnuo YN-300 air pro led light. About £50 with some batteries. Never regretted it and comes in handy in my studio as well.