I just saw this, sorry for the late response.
My son had torticollis which resulted in plagio and we opted for the helmet. It was the best thing we ever did. We went through cranial technologies. I think their website was www.cranialtech.com . There are also things like STARbands, and I'm sure since 2004, there's even more options.
PM me if you have any other questions. My son wore his from 5-8 months. It corrected his ears being assymetrical, his head basically looked like a wedge. It was really bad, and that was just at 5 months.
It does not always correct itself -- especially if the child has torticollis that contributes to it. If the child does not have a contributing condition, I would say that it could possibly correct itself - or not get any worse.
We were proactive with my son's, and I'd do the same thing over again if it happened. I'm not a fan of "watchful waiting" when my gut screams something else.
Luckily, my daughter did not have the tort and preferred sleeping differently, so we did not have to deal with it again.
Just remember, it's not your fault.