It depends on what you do: flashing different firmwares is risky and if something goes wrong you can brick it or you can have some loss . For example model number or unit ID loss. NV backup preserves to trash your bricked unit or to ask for help to revive it (if possible)
I think if you flash something different in Nuvi from what is standard, you must have a bit of knowledge about what are you going to do. If you have it ,you surely backup your Nuvi NV

Uh kunix was faster lol