The simple answer is no! Just adopt a regular seated position with a straight back, feet flat on the floor. However, if you prefer to use the posture of a meditation practice that you are familiar with (lotus/half lotus, meditation cushion), that is fine too.
As long as the posture is stable, with a straight back, the meditation system will work for you. We don’t recommend lying down during the meditation – it is too easy to drift off to sleep, and you’ll lost some of the benefits. But you’ll get a great sleep, so not a complete loss 🙂