It really depends... when I decided to come out, I planned every detail so that none of it would end up in my parents' ears. I started telling people as I was finishing up high school, but never even thought for a second that I should tell my parents. They said horrible and often ignorant, clueless things about gay people that made me understand they would make the rash mistake of disowning me if they were to find out I was gay, only to regret later. As much as I valued honesty and was not convinced by the concept of white lies, I figured it was a win-win in a big picture to hide the truth from my parents until the time was right.
I came out to them over a year ago, when the time did seem right to tell them. This "right time to tell" is, unfortunately, different for every person. If anything, you probably already know the answer. No one will know how your folks feel about homosexuality better than you do, since I am sure you've listened in on them more closely than anyone if the subject was ever brought up around them. Based on what you know, decide whether to tell them or not on the basis of pure logic and selfishness. Be aware of what and who you need. Be as cold and ruthless as possible when evaluating your parents on how they may react to you coming out. Are they just as idiotic as those bigots who think God created AIDS to purge gay people? Have they made any decisions in their past that seemed rash and careless? Can they become easily blinded from what's really important if something they falsely believed for so long comes under question?
If they accept you for who you are, and tell you that they still love you... well, if your parents were never the liberal type, it's still going to be kind of awkward. Your parents will start talking about grandchildren, then look away uncomfortably. Your parents will start talking about this cute girl they were introduced to by their acquaintances then sort of shrug it off. The transition won't be smooth, and you will have to give them time to adapt. Be as understanding and sympathetic as possible.
I personally am happy that I came out to my parents. The past year helped me grasp just how much they love me and also need me to be strong and happy. This, in turn, helped me build a stronger position against triggers and mood swings. I can't possibly imagine myself being alive today if I had not come out to them a year ago.