If the body is overall relaxed, the mind will eventually follow. A few minutes establishing physical comfort means you’re more likely to enter a deep state of meditation, and quicker.
It’s a foundation; a brief preparation that pays off throughout your meditation.
Sitting upright is best – not in a tense look-at-how-straight-my-back-is-way. Use cushions against a wall, for example. Or sit on a comfy chair, legs uncrossed, feet flat on the floor.
And, yes, it’s fine to change position if you get a dead leg, for example, half way through. Move your leg mindfully and it becomes simply a part of your meditation; the art of witnessing.
Step 2 of The 7 Seven Steps of Calm talks you through finding the best posture.