I guess that if both Shioda and Chiba used that phrase the only conclusion must be that it came from O-Sensei!
I find if I let my fingers open when drinking a pint I just tend to waste beer...
Just posted above - this is standard in Daito-ryu, and was one of Takeda's common statements.
O-Sensei said it as well, I can't remember the exact quotes off-hand.
If you look at Kisshomaru, his fingers were almost always open in this way.
As you get further away, you notice that a lot of folks have let this practice slide.