Yes, it varies by state. Review the posting requirements for your own state.
It sounds like you're ready to go overboard.
"How many feet apart?" Perhaps every fence post? That way, someone can't claim they didn't see it.
My "purple stripes" were put up on one side partially adjoining a mobile home park. I put them facing the three roadways which were perpendicular to my property line. The only reason I did that was I found a lot of trash along the line the first time I mowed. By trash, I mean hard trash. Like cinder block, wood, car parts, furniture and old toys. I also told the owner of the park I was going to put a hog pen along that line if the trash continues. I haven't had any more problems.