A nand backup is of course recommended; but with
a proper dumper instead of creating a rednand just to back that up (when the first CFWs based on precompiled fw.img were invented, we didn't know any better; but the rednand format drops the ECC data from the slc/slccmp partitions, meaning those backups are useless for flashing to sysnand)
(It's actually possible to recreate the ECC areas, but since we now can simply do proper dumps...)
(There's no software nand/eeprom restorer either, the earliest possibility for homebrew code execution is with CBHC, which is still extremely late in the boot chain compared to BootMii on boot2)
In addition to the raw nand+eeprom+otp backups, it's a good idea to
also do a file-level backup (using FT2SD or ftpiiu-everywhere), since those can actually be restored as long as you can get back into ftpiiu-everywhere (which means, for one, almost every Vwii brick)