To be fair, the jiggliness will vary a lot between machines. A trike jiggles you on two axes, wheras a bike only jiggles you forward/backwards, and large wheels and/or good suspension can make a hell of a difference. The same sort of issues we discussed in the mirror thread, in fact.
While I don't feel a need for a headrest, I can slip down in the seat on the Streetmachine and rest my skull against the top of the seat without too much brain-shaking. I wouldn't want to do that sort of thing on barakta's Sprint (admittedly only the rear suspension model), as it's much shakier. Kevin urged us to get a headrest for the trike, but we decided that having a slightly more comfy armchair wasn't worth the extra width while carrying it through doorways sideways. Barakta has since discovered that a rolled up jersey or jacket wedged between the top of the seat and her Carradice rack bag makes a reasonable headrest while stationary, anyway.