It is a bit complicated for me, I told my wife about 4 years into our marriage I have Gender Identity issues, but she didn't seem to say or care much. Apparently she didn't understand what I was getting at, or rather, didn't understand the scope of the problem. It wasn't until she was unfaithful, and things were basically over that I explained to her in detail the problem and that I wanted to transition.
She has been supportive since then, but I still don't know if there is any salvaging our marriage but, that doesn't really have to do so much with me being trans, in fact she seems to like that fact, at least at the moment but I do think that once it goes farther she may have trouble dealing with it, but we will see.
Based on that, I have to say I should have told her earlier, but certainly not before we were married. I just get the feeling, that while she is supportive, that it would have made her less interested in pursuing a life together, knowing that I was "damaged goods" and that I had these problems. I also didn't understand that I really needed to transition until a few years ago, I thought that I could beat it, and that would have been horrible trying to get her to support me "beating" GID.
Honestly, I am not sure what my policy for telling people, or even if I will tell people if my marriage finally ends. I mean, at a certain point, I have an obligation to tell them my future (I can't have kids, periods, etc) but, I don't feel a person has an obligation to the past. Though, that is rather hypothetical since, at this point, the vast majority of my transition is in the future, not the past.