I've been going through the same issue, as I have an exhibit currently on view at an area creative arts center here in Maryland. Some of my pieces are titled, but some are just 'Untitled #__'. If I title comes to me, then I'll use it, if it doesn't it doesn't. Sometimes someone will suggest a title to me, and if it strikes me as being the right one, then I might change an 'Untitled' to that title. I'm completely flexible!
The one thing that I do have against titles is that I don't want to unnecessarily 'steer' someone into seeing a photo in a particular way -- I'd like people to create their own narrative, which may be entirely different from my intention, or from what was actually happening at the time I took the photo.
For example, I currently have this photo in the exhibition, and it's 'Untitled':
Untitled by
Vince.Lupo, on Flickr
Now, a friend of mine suggested that I change the title of this photo to 'Salem', and I actually like that title. But, if I did that, then it would put you in mind of 'witches' or something occultish. However, if I called this photo 'Midnight Meeting -- Jackson, Mississippi', it might put you in mind of something completely different. Of course, the reality of what was actually happening at the time I took the photo was far removed from both of those premises, so that's why I have it 'Untitled'. Let the viewer make up their own story.
The one pet peeve I have is to incorporate the name of the place in which the photo was taken in order to (in my view, anyways) 'heighten' the significance of the photo, when in fact the photo is kinda boring either way. Like 'Chairs, Paris' or something like that. If it was just 'Chairs', it might be just an uninteresting photo of some chairs. But add the word 'Paris' in there, and all of a sudden there's this connotation of romance and all the mythical air that's associated with the city of Paris (admittedly I am coming at it from a North American perspective), and now the chairs take on more significance, when in fact it's still an uninteresting photo of some chairs (and in reality those chairs may have just been sitting on your front porch, and nowhere near Paris, France). Having said that, people at the reception for my exhibit were wanting to know where a number of the photos were taken. I generally obliged and told them, but for me it was affecting the way people 'read' my photos. Whether it was good or bad, helped or hindered, well that's not for me to say. Personally I prefer that people not know, as I don't think it's important to my particular work. Then again, I'm not the viewer, so I could be wrong!
So I'm still wrestling with the issue myself. I think that if a title comes to you, great. If not, 'Untitled' #__' is fine too. And if a photo starts off as 'Untitled #__' and then a title comes to you later, so be it. All depends on how much you want to influence people's interpretation of the photo.