Considering the explanation in Terminator about why the Terminator killed those other Sarah Connors, I don't believe Skynet would have even known where to begin with trying to kill John's grandparents. And anyone who knows timetravel knows you don't just go back and kill everything in sight. Skynet could end up ensuring that it never gets created.
If you knew about time travel, you would know that if Skynet never got created then nothing would be sent back in time stop it from being created. Of course the most important rule of time travel is never go back in time so that you meet yourself because that would be embarrassing. The second most important rule is never send a chicken back in time because if it laid an egg you would never know which came first.