Overdoing it is when temperatures exceed 80C. Anything below that and iz aall goodh. But increasing the core clocks doesn't increase temps; it's when you up the voltage that temperatures start to rise.
Increase the core clock by say, 50 Mhz. Apply settings. Run your game. If it crashes, then reduce it by 10 Mhz. Repeat until it crashes no more. But if it doesn't crash 50 Mhz right off the bat, then slowly increase by increments of 10Mhz. You should be able to get it to 1070 Mhz without any voltage increase. 1100 even.
If all you want is a slightl overclock, then I recommend the above i.e. any voltage increases. 1070 Mhz is still a 15% overclock, which is quite good.
But remember that every GPU is different, and what you have to do is find the optimal core:voltage on your GPU.
This guide should help you:
http://www.anandtech.com/show/6359/the-nvidia-geforce-gtx-650-ti-review/20
Hope this helps.