I came out to my mom when I was driving down a country road, so that if there was a problem or she didn't want to confront the situation she had no choice, but then again I'm a little twisted.
I personally wouldn't go for a letter, unless you don't live with them and are afraid they'd be violent toward you. If that's the case (or any case where violence is a possibility) I would put coming out on hold. I'm assuming your a minor, cause if you weren't you could apply for one of the binder charities without need to come out first.
Once again, I don't know you, or your situation. What I can say though, is if you want your parents to take you seriously, having the courage and self esteem to say it to their faces will do wonders.