It's easy and use ASUS EZ Flash 2 as mentioned - http://www.tomshardware.com/forum/289507-30-what-flash
In short, if you have no problems then my best advice is NOT to flash.
Examples 'when to flash':
1. CPU requires newer BIOS version
2. Non-OS Instability
3. Peripherals e.g. 2TB HDD requires new BIOS.
4. Memory compatibility ; where current RAM is causing errors or stability.