This video has been posted before on these forums but at 4:40 he starts talking about the "proper" way to hold a knife. It's pretty much what Mark said.
(The whole clip is worth a look. I think it puts aikido tanto training in context).
++1 for you I think it puts not just Aikido tanto training in context but a whole bunch of other arts in context too, i.e. the crescent kick at the forearm knife disarm.
Have integrity in your training and place all your training into proper context.