History:
The practice of "topping out" a new building can be traced to the ancient Scandinavian religious practice of placing a tree on the top of a new building to appease the tree-dwelling spirits of their ancestors that had been displaced. The practice migrated to England with Scandinavian invaders and took root there.
Topping off ceremony:
Today, a non-religious but formal ceremony is often held to commemorate this milestone in the construction of a building. All tradesman on the job usually join in the celebration as well as the supervisors, representatives of the architecture and engineering firms, the owner or representatives of the owning organization, donors, and any VIPs that are invited. The ceremony is often parlayed into a media event for public relations purposes.
While the ceremony itself has no standard agenda, it usually includes the placing of an evergreen tree upon the structure to symbolize growth and bring luck. State and national flags are often raised atop the structure. It may take place during lunch time and can include a catered meal and entertainment. In large building construction, the topping out beam may be signed by the ironworker crew, or by local dignitaries depending on the importance of the building.
The topping out ceremony is similar to ship naming and launching ceremonies and probably of similar antiquity, and was perhaps done to placate the gods and to shield the building from harm. http://en.wikipedia.org/wiki/Topping_out
From the NYT: http://www.nytimes.com/1984/10/21/realestate/the-hoary-tradition-of-topping-out.html?pagewanted=all
From Columbia University: http://www.columbia.edu/cu/ssw/news/apr03/history.html