I know that this is an old old topic but I just started posting the other day and thought that I would put my 2 cents in on the subject ;D
We started with 2 balas at first in our tank and they used to do this rubbing thing...but at first it was only after eating (bloodworm feedings...not the brine shrimp feedings) At the time they were pretty close in size...but once the one got a little bigger...they started doing it after every feeding of frozen food. At first I thought that it was a mating/dating type thing (because one being smaller than the other) We added a 3rd bala (who was comparable in size to the bigger original bala) and we noticed that within 2-3 feedings the two larger balas started displaying this behavior.
I have now noticed that since we have moved the balas to a bigger tank...they have yet to display this rubbing behavior. Kinda saddens me though...because it was REALLY cool to watch!!!!