I don't know about keyboards, but I do know that it's customary to coil the Selectric power cord around the two platen knobs. This prevents the roller from shifting around as well as secures the inner assembly to the case. It also prevents the cable from being accidentally tugged, which could damage the power switch box inside, as it's not otherwise retained... When I shipped my 3278, I wrapped the cable around the top part of the keyboard horizontally. :D
I looked around for a photo of a Selectric as you described, and was relieved not to find any. I believe you, that it's a good shipping precaution—but take
photos of it that way, and destroy those beautiful lines? Yikes. Anyone who does that is heading back to Online Marketing 101.
And speaking of shipping, it's especially goofy when people show
detachable cables wrapped around keyboards, like this Model M. Failing to detach it and wrap it separately leaves the connected end unnecessarily vulnerable.
And yes, definitely, there's what Findcanor just pointed out. Some keyboards—e.g. Dell AT101's/101W's—are so reactive to cable contact, it's practically impossible to find examples without long scars melted into them.