Be advised that this knot should *NEVER* be used for ropes of different diameters. It will tend to roll apart under those conditions. I had a guide tell me there was a death because of this (I have not been able to comfirm this). As I understand it, the greater the difference in diameters, the greater the tendency to roll apart is.
I was always very suspicious of this knot but over the years have become more comfortable with it. I now use it about 1/2 the time. In general, I tend to like knots that want to tighten under force (rewoven knots for example), this knot will tend to pull apart under force which is why I am suspicious of it.
With that said, if you have 12" of tail, nothing is going to happen and if the rappel is straight forward, why make yourself undo a rewoven knot.
Also, if you have 2 different diameter ropes and are committed to this knot, I was told that you can put 2 knots in back to back and it will solve the problem. I've never tested that though and am not sure I ever will.