Hi, I came out to my mother around the time I started therapy. She passed it on to my sisters, an aunt and uncle, and of course my step-father. I was dressing when I had the opportunity to do so, which wasn't often, had not yet started electrolysis (which I've since discontinued for the time being), and was not on hormones. I did it because I was in a bad place emotionally and I needed someone close to know what was happening to me. I already knew I was female and didn't feel I needed to be in any particular phase of transition before coming out to them. I don't see a reason to wait, in my opinion. We are who and what we are, if not physically, then mentally. I think the sooner they are aware of your true gender, regardless of your appearance, the sooner you may attain their support which you will want as you journey through transition. Again, just my opinion. Also, I am by no means suggesting that they will be supportive. That varies with us. I was fortunate enough to have a very supportive family (took my step-father a little longer), but I know not everyone will have the same kind of support I experienced.
Katiej does bring up a valid point regarding family being educated on this. After coming out to my mother, I sent her a book regarding transsexualism, what it is, etc. A few weeks later she sent it back to me with writing in the margins and highlighted sections. She made a point of highlighting and commenting on sections that dealt with being supportive, which really touched me. As I mentioned previously, not all family members may be supportive and I don't think it matters what phase of transition you are in. Showing up at home with makeup, your best dress, and nice little breasts won't make it any easier than telling them about it before transition. Again, just my opinion.