Thanks, Balletboy and Dr. D.
It was a stupid question, or maybe the way I phrased it was stupid. I realize now that all of the weight should be on the front, supporting leg, and the rear foot should be held in arched position.
I just got so tired while repeating this that I did in fact start to shift my weight to my rear leg and foot.
The B+ position as used in my class that night was indeed a starting pose, from which the rear foot commences an assemble, leading into waltz steps.
[/i]