As a rhythm musician, I don't let the front line speed up unless I know they're doing it deliberately. Watching dancers is ok, so long as you remember that some parts of the anatomy have a different frequency of sympathetic resonance than others, and if the dance floor's crowded the people are going to end up shuffling tneir feet which is a lot harder to do in time than dancing. As for metronomes (or drum machines) they can't cope with the nuances that you want when playing for dancers.