I ran my previous GTS for over a year overclocked. I kept it around 210 Core/384 memory for daily use(gaming etc.). It's upper limit was around 245/398. Pretty good OCer for a 32MB GTS. I didn't like the core up that high though because it just had stock cooling.
So in fewer words, yes it's perfectly fine to OC your card. As long as it is stable at the higher speed, it is virtually the same as running it at the stock speed(in regards to it dying due to use).