Istanbul is a massive metropol with about 15+ million people living in it. In my opinion, there is no specific answer to this question. My answer would be different depending on the situation, neighborhood, etc. I have done it in the past, and it was definitely safe for me.
First of all, the public transportation in Istanbul is one of the best in Europe. You can commute 24/7, if you don't need to go too far from the central areas. So, I would definitely suggest taking the public bus, minibus. Cabs are pretty cheap comparison to other countries.
If you still need to hitchike, then just walk to somewhere that you can see the driver of the car, maybe a traffic light. If you are a woman, then definitely don't hitchike by yourself. It doesn't worth the risk.
