I do it a lot. It's a giveaway that I'm approaching or at some level of mania. But I have had to learn in the last year to not sit with my toes pressing into the floor, heel up, ready to start bouncing (I think I do it when I'm deep in thought as well) because it was making my achilles tendon get very tight. It was partly from an injury and partly from doing that. It was easy to tell how much I did it; all my shoes were worn out with the toes bent up a bit from being in that position.