Yes, you should worry about off-axis since that will reflect off the side walls to your listening position in the center, but you're under a misapprehension as to the vertical center. The main reason for the suggestion(besides being an exact match for the mains)is that a vertical center generally has wider and smoother horizontal dispersion than a horizontal center. If anything, listening from directly in the center on the bed might suggest using no center speaker and allowing the mains to handle the "phantom" center channel, since the shift of center image for off-center listeners wouldn't be a factor.
