As far as goes breeding, I've only seen tail wagging when my girls were receptive. Often they will push out their scent glands a bit, so I can easily believe that the tail wagging is meant to spread sex pheromones to help stimulate breeding responses from males.

On the other hand, tail wagging can be a sign of stress as well. Juveniles will do it quite often when they're caught off guard, as was mentioned above. I've also seen my adults tail wag for a brief second when I've abruptly opened their tubs. Some people argue that tail wagging during breeding is an indicator of stress as well, but I'm not with this group, as I haven't observed any other signs of stress in tail-wagging breeding females. Anywho, I have a good deal of fun watching tail wagging when it's not stress related, it's very interesting breeding behavior in my opinion. You should see Leo's tail wag, haha, it's much more furious.

Cheers,
-Matt