Selected answers from the Dumb SEO Questions G+ community.
Richard Hearne: Homepage is fine. I would not expect to gain too much from adding this. It won`t hurt, but unlikely to draw more traffic.
Bill Hartzer: I would add it to all pages. You can also add other schema markup code as well, such as webpage schema and organization schema as well.
Scott Hendison: I used to put it in the footer, but a couple years ago started putting it only on the home page and the contact page, after reading somewhere(?) that John Mueller said it shouldn`t be sitewide
Adam J. Humphreys: Scott Hendison ya that and in the header for rendering reasons is preferred.
Richard Hearne: It`s not that it shouldn`t be sitewide. It`s that it should reference a single unique URI, usually your home or about page. Some sites were adding it to every page and not correctly using URIs, so it`s quite fine to add to every page so long as you always assign the same URI to the entity referenced on all those pages. It`s quite technical, so just adding to your homepage overcomes this particular issue.