Since you can save the D3 settings to a CF card, that's probably the best way to keep things in line. HOWEVER, if you save it on the card you use for taking pix, a reformat will wipe out the settings. Therefore I think it's useful to have the spreadsheet as a backup.
I saved the settings to my CF card, then transferred the file to my computer to back it up. I'm kind of on the lookout for a really cheap CF card that I can save the settings to and keep in my camera kit. I'd prefer a small size memory card so I won't be tempted to use it to take pictures on. 128 MBytes or less would be nice.