Re: Bokken - from the spine or not?
Iwama aiki-ken in my experience
Ichi no suburi: From the spine. In ichi no suburi I have been told specifically to stretch all the down to the spine as the first suburi is also a warm up and a stretch. Combine that with the possibility to check your center alignment by aligning the bokken with the spine. Focus on breathing, center and sinking the hips in the cut.
Ni no suburi: From the spine, although probably not always. When i focus the suburi from the hips I tend to drop the bokken to the spine. Creates a kind of whipping motion/feeling.
go no suburi: I haven't been instructed to align the bokken with the spine in the fifth suburi but I have seen ex Pat Hendricks sensei do it, and it seems to be standard practice fore those that follow H. Saito sensei.
And in kumitachi the general rule is: no dropping of the bokken to the spine, more of a 45 degree angle.