Wait until you are probably positive you are. Find yourself before and make sure you are okay with it first so that other people's judgements don't come on too strong. Unless you think you need someone (ie. your parents) for support.
You should tell them... they are YOUR PARENTS. They should love you for who you are not for who they want you to be. If they cant just remember..you have all the people in the world around you that will(: Your parents should support it... but if they dont it doesnt make them bad people. Just remember just because your parents dont accept doesnt mean everyone else wont either(: BE WHO YOU ARE NOT WHAT PEOPLE WANT YOU TO BE.
If there your parents they should except you no matter what you are rather if be straight or Bi. You are who you are, no one can change that. If your not sure, don't say anything. I'm straight but I think that some girls are beautiful, doesn't mean I'm Bi. You can also have just strong feelings for your friend, like you said, he's your best friend. If you turn out to be Bi, don't be afraid to show your true colors.
You say you might be. Sounds like you're trying to sort that out. I don't think it's necessary to tell them something about yourself that might change. No sense stressing you and them out. When you are confident in your decision ( after experiencing life and growing up ) there will be time to tell them . If ,however ,you feel their input on the subject would be helpful you could confide in them now.