Does it want to update when you start multiplayer for the first time?
[SPOILER="Yes"] Then it is normal, the update fixes some bugs found ingame like lagshooting, poor hit detection and ingame glitches therefore its safe to update [/SPOILER]
[SPOILER="No, It says you need to update to the latest System Menu when starting MW3 channel"] It is not safe to update [/SPOILER]
I have Mw3 for the wii aswell and it asked me for an update when I started multiplayer for the first time. It didn't brick my softmodded wii.