In my Aikido practice we are also taught to grip the sword with our index fingers extended, at least 'for the first 10 years or so'...after which, Sensei says, we are allowed to let them curl down a little bit.
It may sound a little goofy, but I've never questioned this. I guess it's all about focusing Ki, but to be honest I'm not at all sure. What I _do_ know is that we are told to extend our index fingers for taiho as well. Among other reasons, Sensei has mentioned that gripping without the index is perceived as a less-threatening matter by our partners, and therefore 'more Aiki'. I have also noticed in myself a distinct disinclination to grip _anything_ with my index fingers nowadays...which looks a little odd when I'm driving or riding my bike.
David Anderson
Calgary, Alberta
"Swords are the rosary of Aikido"
D. H. Skoyles Sensei 04/03/01
Nakayamakai KoAikido dojo