Should I avoid using Mindfulness to fall asleep? Is the use of Mindfulness of breath for falling asleep a "bad" practice? Might such a practice invite torpor during sitting meditation?
If you are using meditation to go to sleep:


You are inducing a hindrance. If this becomes habitual it will block your progress.
Also when meditating you should be alert and hence reduce the hindrances and sleepiness. If you are not falling asleep it not an issue if you are meditating. It is said a Yogi never sleeps.