Above all else, titles should not limit the viewers experience of the photograph. Usually a title naming the object(s) in the photograph says that that is all the photograph is about. The viewer is directed not to see anything more than that. Other times, use of "Untitled" gets in the way of the viewer's experience of the work--if they are wondering where something was made, for example.
In an exhibition I once saw there was a Walker Evans photograph of a church in Albama. The title was simply, "Alabama." Someone else may have titled the photograph, "Church in XYZ Alabama." To my way of thinking the first title implied that this was a photograph of a whole culture. As such, it expands the viewer's experience of the picture. The second title forces the viewer not to make a connection like that and, in this regard, would be not as felicitous a title.
The worst titles are the "cute" ones and titles like "sunset." They direct the viewer's experience rather than giving the viewer credit for getting something more out of the picture, perhaps even more than the artist intended.
All photographs that are works of art are about something more than what they are of. And what they are about may be something different to each viewer, for each viewer brings his or her own life experiences to the activity of looking. For the viewer, the art of seeing, of receiving, is a participation in the creative process no less essential and direct than the artistís own. Do not limit that experience with poorly chosen titles.