The i7-4770K can achieve 4.2GHz usually without changing the Voltage or otherwise messing with the power management. Once you change the Voltage the temperature can go up significantly requiring better cooling (or more noise with same cooler).
Since real-world gaming is barely affected by the difference between 4.2GHz and 4.5GHz I advise 4.2GHz.
Here's the AI Suite 3 manual for tuning:
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/MAXIMUS-VI-EXTREME/DE138_ROG_AI_Suite3.pdf
My quick advice:
1. Flash BIOS to latest, using USB BIOS FLASHBACK prior to installing Windows
2. Verify the DDR3 memory using memtest www.memtest.org (at least one full pass)
3. Install Windows
4. Install motherboard MAIN CHIPSET driver, other drivers and software
5. TUNE CPU in BIOS or via the AI Suite 3 software so all cores are 4.2GHz ( x42 multiplier of 100MHz base clock)
6. CONFIRM that the memory is also proper (i.e. 2133MHz, not 1600MHz if you have 2133MHz memory)
7. Tweak the FAN PROFILE for every fan attached to the motherboard
8. Rerun Memtest
9. Run Prime95
That's the basics for tweaking and testing the CPU and Memory though read your manual fairly well.