Originally posted by Adam The 6x7 isn't meant to be hand-held, since that would sort of be a waste of film.
Adam, do you have a 67? Fortunately, the OP can safely ignore that advice.
Yes, you can can handhold a shot. How slow? Go too slow and you'll get camera movement just like a 135 film camera. It all depends how well you hold the camera.
I've shot as low as 1/30 with a wide angle before. It is easier to do with a WLF than a with prism on for this technique. You hang the camera from your neck applying some tension to the camera straps with your body bent over slightly pushing down on the camera. You can move the camera around for composition while looking down through the view finder. You need to have already focused first. Lock up the mirror if you want and take the shot.
I second RawheaD's comment on the wood grip. I stopped using that thing 20 years ago. If weight is an issue, get the folding hood. It does cut a lot of weight out. But it is problematic for handheld portrait orientation.