Yes. Your brain changes from moment to moment and the one-minute calibration process is a required step to make sure Muse will provide accurate and responsive feedback.
The results of a Muse session is dependent on the quality of the calibration. This stage is important as it provides Muse with a picture of your active brain to compare against during the session.
Muse is a very sensitive device and so will pick up even the small electrical activity from muscles around the eyes, jaw clenches, and also the brainwaves from visual processing. Therefore it's best to keep your eyes lightly closed throughout your calibration and meditation sessions.