Yes, there's a reason. The lotus posture and related positions are meant to close off your body's interaction with the earth energies by turning the soles of your feet upward. That's important in those Asian systems of spiritual practice that focus on cutting your ties to the world of manifestation. In most Western systems of practice, by contrast, the goal is to be able to mediate between the spiritual and material worlds, and so your body needs to be connected to both sets of energies -- the solar current, to use Druid jargon for a moment, through the crown of the head held upright, and the telluric current, through the soles of the feet pressed gently against the ground.

Still, it's up to you which posture to choose.