Personally I advise against it. There's a definite risk of bricking your router if anything went wrong during the update. Plus there is little benefit in doing so when running Asuswrt-Merlin - it's mostly useful when running a custom firmware that cannot address the whole 64 KB of nvram, like DD-WRT.
Asuswrt-Merlin: Customized firmware for Asus routers
Github: github.com/RMerl - Twitter: RMerlinDev
See the sticky post for more info.