I commuted a long length of the central line in my first pregnancy and used my badge from about 7 weeks when I started feeling rough. Sometimes towards the end I didn't wear it because it was more comfy to stand up as he engaged.
I never had a problem getting a seat, I would walk through the seats and hold on to the middle pole and people were pretty quick to notice when I started to show.
I do think the central line is pretty friendly though!
People seem to get a bit irate about these badges, I'd just ignore it from my tube seat and wear it when you like.
When I was about 18 weeks with dc2 I had a terrible bleed and had to travel into London by train a few days later and could have kicked myself for not having it with me. The train was boiling hot and absolutely rammed and a seat would have been great.