thinking of comming out of the weed closet.....

Personally I think it would be better to let them find out on there own. You dont have to go out of your way to let them know you smoke.

I think you should tell them. If smoking means that much to you, then should be a part of your life you want to share with your parents. I'm sure you're parents would like to know your interests, and if smoking is one of them, why hide it? I doubt at 20 your parents can have that much control over you, you've probably proved yourself responsible, so of course they won't mind right?
I do think you should tell them. Whats the harm?