I'm usually an index finger person, unless I'm using the Technika with the grip that has a thumb release--

http://www.linhof.de/english/gallery/sport.html

The 5x7" Press Graflex also has a thumb trigger of a different sort. The fingers of the left hand are supporting the camera with the thumb on the trigger. The right fingers also supports the camera with the thumb on the focus knob. I don't know if it's relevant, but the Technika and Press Graflex are both left-hand releases. On the other hand, so to speak, I have a left hand L-grip for my Bronica S2A, but it's an index finger release.

With my F-1N, I use the vertical release on the motor drive. With the Vitessa-L, the thumb stays on the focus wheel, so still an index finger camera. I don't usually turn my 6x6 cameras sideways, unless I'm doing something like supporting them against a wall or a post for a long handheld exposure.