This is definitely an individual choice. i have had several miscarriages. I can tell you, from experience, that waiting to tell people is hard. my husband and I chose to do that for my son, and this current pregnancy. With my son, we waited until a third ultrasound, which was about 11 weeks. For this pregnancy, we waited until i was about 10 weeks. With the others, we just told them that we lost the baby, which was also difficult.
i have a friend, who has had several miscarriages, who still tells people right away, only because she wants people to know why she is acting the way she is. She also figures if something happens she can tell one person, and they can tell the rest.
What was really hard for me, all times, was that i had no one to talk to. The only ones who knew were my husban and doctor and every time i felt llike anything was wrong, I couldn't tell anyone. My husband, although extremely supportive and sympathetic (and in the medical field), just didn't truly understnad my anxiety, so he was not that much of a help. Luckily, my doctor's office doesn't think I'm too crazy, and they answer all of my questions.
So, i guess, you have to do what is best for you. If you need someone to talk to, i would tell someone. Like I said, it's not any easier not telling, because you would have to tell others about the loss anyway (which, I will keep my fingers crossed, will not happen again).
Sorry this was so long. Good luck with your decision.
T.