Mate,
A "safe" parameter to consider would have to involve both Voltage and Temprature. It is the combination of the two, which damages the CPU. With a stock cooler, I would be loath to use anything beyond 1.525V. As you will see, the temprature would rise rapidly, often hovering around the 44C mark. This to me, is a bit risky on stock cooling (I have used stock cooling in the past and have found it very inconsistent with regards to the ability to maintain a constant temprature when overclocking). However, a lot depends on your ambient temprature as well. As long as you can keep the average temprature below 42 C (the number I'm comfortable with), feel free to experiment with the voltage.
I believe the absolute max temp rating for your processor should be around 52 C. You might want to grab theCoolest's nifty little program to find out for yourself.
Good Luck!
S-N