This is a bit off-topic, and is probably a naïve question, so forgive me in advance for both. I've always been under the impression that "engineer" was a protected professional title, just like "medical doctor" or "lawyer", at least in part because there are certain legal responsibilities that come with the title. It's thus a little confusing to see the title being used, here on HN and elsewhere, by self-taught folks…
I think it only matters if you're offering services to the public as an engineer, or if you're doing public-works projects. Then you need to be licensed as a Professional Engineer.