Well, with my deaf sister, the only thing we needed to get a sign placed was for her to get hit by a truck while playing one day. Aside from needing several stitches and getting lots of bumps and bruises, she was fine.
I doubt my parents threatened to sue, but it wasn't until that incident that the sign went up. I know they requested it beforehand.
So, based on my experience - I would say that if you know there is a deaf child in the area - PUT THE SIGN UP. Pretty simple.