i am using intel core i7 2630qm
Actually, it is preferred to use all cores.
Why do you ask?
cuz i thought it will damge the processor or increase its temp
No, that is not the case. Provided that your cooling solution is keeping the temperature at or below, say, 90c, you will not see any degradation of the processor and it should work for its full warranted lifetime.
For more complete information about compiler optimizations, see our Optimization Notice.