It depends on the dancer's skill level and desires. Paradoxically, the really expert competitive dancers often like their music slower (sometimes to a ridiculous degree) than the relative beginners. This is because the experts' footwork is a great deal more complicated than the beginners, and they need more time to work it all in.

When I'm playing, I just naturally want to play at my normal dancing speed, if I start the tune.

130-150 beats per minute (for jigs) works fairly well for me in dancing (gives away my relative skill level, doesn't it?). Reels I can manage up to about 180 bpm, but I will be huffing and puffing.

The best rule of thumb, for me: if you can do your ornaments cleanly, you're not playing too fast. If you're dropping out ornaments because of the speed, you need to slow down.

J-t-T