I would not deface signs or steal them from private or public property, and I would not put up signs of my own on private property I didn't own, but I would put up signs of my own on public property.

I did that in grad school, actually; someone put up a bunch of signs in the humanities building advertising the next meeting of the Young Socialists, so I printed up signs advertising the next meeting of the Objectivist club and taped them up right next to the socialist meeting signs.