As far as I know, Nintendo has never done a firmware update that doesn't first warn you it's about to update. I wouldn't say it's optional, because games will refuse to run without them, but of course you can work around that.
There are indeed some updates that do only harm (possibly bricking even an unmodified Wii, for no gain), and you are better off not installing them... since that particular update would also wipe out BootMii, the only safe thing to do is use a homebrew loader to bypass installing it. Silly, silly Nintendo.
is now live (but may take a few tries to load)
currently down; check out my PSP/DS/Game Boy/Windows/Linux homebrew, ROM hacks, and Gameshark codes!