How much does mouth breathing at night matter?

It can matter quite a lot, mainly as a symptom rather than a problem in itself. Habitual mouth breathing during sleep is associated with dry mouth, dental problems, poorer sleep quality, and it is often a marker of nasal obstruction or, more importantly, of obstructive sleep apnoea. If you wake with a dry mouth, snore loudly, or feel unrefreshed despite adequate hours, that combination is worth a medical assessment rather than a taping trick. Address the cause first — allergies, a deviated septum, or apnoea — since forcing the mouth shut without treating obstruction is not sensible.