With that max temp don't even think about overclocking on stock cooling.
Anyways to answer your question technically it is possible, but we all strongly advice against it as it will shorten the lifespan of your cpu significantly (could even just burn on the spot). Overclocking in your scenario would probably be on your top 10 list of most stupid things you will have ever done, unless you take part in "Jackass" type stunts.
I have an i5-2500K that's idling around 40C. I ran RealTemp and it said the max temp was 98C. Should I be safe overlcocking on an Asus P8Z68-V LX?
40c sounds a bit high. Are you certain that you have all 4 pushpins solidly through the motherboard and locked?
Regardless, yes, you can hav a conservative oc with the stock cooler. It will spin up and get noisy under load.
If the temperatures get too hot, it will throtle down to protect itself.
By conservative oc, I mean just raise the multiplier, and leave the voltage on auto.
Old tech but I was running my E7500 at 3.6Ghz on a stock cooler and thermal pad full load temps never got over 72C. The stock cooler would not work at 3.8 though. Maybe I got lucky with my CPU.
If you want to over clock on the stock cooler I would recommend not going over 73C (full load) that seems to be the where the super coolers get to on very large overclocks. And generally seems to be accepted as a safe temperature.
40C at idle sounds a bit high unless your case cooling is really crappy and/or you have an improperly mounted HSF as mentioned.
I have a crappy case cooling story but this isn't the time.
Turns out the reason it was so hot was because the A/C in my house wasn't on and the room temperature was around 85F. I'm now multitasking (6 tabs open on Firefox, Dreamweaver, Photoshop, and downloading a game on Steam) with around 33C.