For me, it depends largely on how thick the rope is that I'm using, single strand vs double strand rap, whether I'm using a back-up or not, and how much weight I'm carrying. Most of the time, I use the low friction side, but will change if I think necessary.
Just curious if anyone has any thoughts on which way to orient the ATC when rapelling. If i were to try to figure it out on my own, i would guess that if you're a bit nervous about losing control, you should use high friction mode. (In addition to a prussik/kliemheist or fireman's belay, of course)