Hey John,
My Ol' man used to say that free advice is worth what you pay for it, so here's my 2¢.
Most planing benches are best situated somewhere between your elbow and the top of your hip. If possible, I recommend buying a six foot pine 1x12, clamp it up in the lowest bench that you can find and start planing the edge. Move the piece up and down to find your best ergonomic.
The problem with using the best ergonomic for planing is that it's usually too low for most other work (read backache waiting to happen). The cure is to use a secondary workbench; a small platform to raise the work to a more comfortable height for detail work.
Another solution that I sometimes use for detail work, is to sit on a pneumatically adjustable drafting chair. They are usually available for a reasonable price from most office supply stores. Because it has a infinite height adjustment, it's usable for a number of workstations. For instance, my bandsaw table is low enough for resawing heavy timber, but too low for my back when doing detail work. The drafting chair makes a comfortable perch.
One other reason I'm fond of a drafting chair in the shop: When I was a small one, my Pop would put me on a tall (I thought it was tall) drafting chair in the center of the shop where I could watch everything, and stay out from under foot. I could stay in the shop as long as I wanted, if I would stay on the chair.
Cheers,
Tom
_________________
"There is no path to peace, peace is the path."
Mohandas K. Ghandi