I use EVEREST Ultimate Edition and It's said that my CPU i5 760 overclock: 5%
I didn't do anything about over-clocking. I want my CPU remain stable. Does it auto-overclock? Does 5% overclock is necessary for my computer?
Your CPU can auto-overclock, sort of. Newer Intel processors have a a feature called turbo boost that will automatically raise clock speed above stock if certain conditions are met. Usually turbo boost kicks in when fewer than four cores are active, or when your CPU is running below it's thermal design power.
You don't have to worry about it becoming unstable due to turbo boost, because turbo boost also adds additional voltage to the cores whose clock speed has been increased so as to prevent instability.