I'd say that's a bad habit since the best way (deepest) of breathing is using your diaphragm/belly, even when walking.
I think there's a tradeoff here. I had the exact same problem as a teen (still, in a way, but not as much) and I was extremely self-conscious about it. I also played the saxophone and had to learn how to breathe properly. I recall reading in a ventriloquism book at the school library -- wow, over 20 years ago now??? dammit I'm old... -- that deep diaphragmatic breathing takes in 2-2.5 times the volume of air as pure chest breathing. Confirmed that ten years later in martial arts.
At the same time you don't want your gut hanging out without any ab tension, because that will cause lower back pain. Again, speaking from experience.
I think if you are subconsciously "sucking your gut in", but at the same time you can breathe through your diaphragm, you should be in balance. I'm no scientist or doctor, but this "feels right" to me. You need the ab tension to help stabilize your upper body for movement, but you also need it to be flexible enough to breathe properly. If you can put your hand on your chest and your belly button, and when you breathe in the hand on your belly button goes out farther than the one on your chest, then you are breathing correctly. If you are still pulling in your stomach while doing this, you should be fine I would think.