Greg, I agree that intuitively it shouldn't matter which knot you use if it doesn't interfere with the sheave BUT in practice I've found that it DOES matter. I used to use a small bowline and had no end of problems (this was with a Tornado). Then Reg White told me to use a stopper instead. Problem solved!! Ever since I've used the same setup on all my cats and never had any issue with it.

Thinking about it, I guess that because the halyard exits the ring on one side (nearest the mast) it forces the ring to fall AWAY from the mast more readily.....