For your spine health, yes, it absolutely matters, if you're having a hard time sitting upright perhaps you need to readjust your posture to find a more comfortable sense of sitting with a straight spine. For the energy flow of the chakras, I believe it would slow the flow but not stop it, however, others might certainly disagree.
When you meditate, it is best to feel as comfortable as possible. Sitting straight up is better for your back but it may make it harder to concentrate on meditation. I enjoy meditating while leaning back in a comfy chair or laying down because it allows me to focus more on my meditation than my posture
no, it doesn't matter as long as your in a comfortable position whether that's sitting or laying down. however, if you are sitting it's beneficial to sit up straight to improve posture which can help with back and neck pains.